I think this conclusion is right, but with certain qualifications to your suggested reasons.
If you're eating mostly beef, then conventional or not, you'll be getting relatively low amounts of o6. Just over 2g a day for 0.5kg of fatty ground beef. My butter contains substantially more n6- 2.7g per 100g. Combining them both, I'm only getting around 5-7g per day anyway.
The possible reduction in o6 from going grassfed is therefore very slight indeed. The absolute increase in o3 is also pretty slight. This article linked to at MDA suggests between 60mg and 100mg per 100g of beef. (Even omega 3 eggs are 14% o6- which I figure to be approximately 5g for 6 eggs- and a ration of 0.44:1 o3:o6, which dwarfs any o3 you'd get from meat. Similarly, 200g of canned salmon contains about 5000mg of o3, which again dwarfs the amounts from grassfed- a week of which would give you 3500mg by those figures).
It's true that grassfed does contain more CLA. Again from the MDA paper, grassfed contained between 15 and 64mg per 100g, compared to 18-63mg for concentrate fed. Even if we think that it's a lot preferable to conventional beef though, the absolute levels are pretty low. A tablespoon of pastured butter is 250mg, for example. Given that I eat about 100g of butter per day, I'm not particularly worried. Indeed, how much butter you eat would seem to be substantially more of a factor than whether you eat grassfed. Even 67% pastured butter (which I hope British butter meets) contains 150mg CLA per tablespoon, so I'd hope that I'm fine (that's more than twice the CLA per 100g for grassfed beef).
Therefore eating conventional beef and eating oily fish seems to me to be at least as good (plausibly better in some cases) than eating grassfed alone.
The couple of qualifications I had were these.
Firstly, fish oil is plausibly quite a bit worse than omega 3 from fresh fish. Studies quoted here suggest that fish produced a 129% rise in EPA and 45% rise in DHA versus 106% and 25% for CLO despite the CLO containing 3g versus only 1.2g in the cooked salmon. Adding this to questions about the degredation of the PUFA in fish oil, the absence of natural micronutrients/antioxidants in fish oil and the importance of the triglyceride form, there are various reasons to think that fish oil might be suboptimal. Fwiw, I still take fish oil and eat canned salmon a couple of times a week, depending on how much I can stomach.
Also it's worth bearing in mind that there might be other thus far undiscovered wonder-chemicals to be discovered that grassfed meat has more of than conventional. Presumably it's a bit higher in micronutrients, but I've never seen anything to suggest that this is biologically significant. We only discovered CLA in 1987, for example. I've got to say that I'm sceptical this will be the case, however.
One of my suspicions is that the preponderance of being pro-grassfed in the paleo community is explained partly by its usefulness in reducing cognitive dissonance and making our diet more socially acceptable. Lots of people report that they still intuitively feel that red meat must be unhealthy for you, so it's no doubt a great boon to be able to quiet our own thoughts and the challenges of others by replying "Ah yes, but this is an all-natural, grassfed cow, not one of your grain-fattened, chemical pumped death cows." It's probably also a factor that the paleo movement is based predominantly in America, where the alternative (feed-lotting) is so much worse, compared to, for example, the UK where pasturing animals a lot of the time is pretty common.
Our reasoning above might also be challenged by insisting that the basic paleo heuristic trumps the specific statistics in this case. Namely, it might be challenged that your average Grok, would have predominantly been eating only grassfed meat and therefore whatever grassfed meat contains must be optimal. Certainly we should be sceptical prima facie about pursuing numbers that would be seemingly impossible to obtain in a paleo context. However, there are reasons to be sceptical in this case. For one thing, it is eminently plausible that we spent a significant amount of evolutionary time eating omega-3 additional to that consumed from grassfed meat. Various people think that we spent quite a lot of time near shorelines or otherwise eating significant quantities of seafood. Similarly, hunter-gatherer populations that do eat larger quantities of omega-3 do seem to gain some benefits- like the inuit. Furthermore, we might also imagine that what produced optimal evolutionary benefits in a paleo context might not produce optimal health in a modern context. It is plausible, for example, that there were benefits to being in a somewhat more inflammatory state in a paleo context than is optimal for long-lived health now. Having a strong inflammatory response (producing a few false positives) when faced with injury or infection might well be a net good in a paleo context, whereas having reduced inflammation, triglycerides, blood clotting etc might well be a boon for optimal health now, where we're more inclined to face chronic inflammation than die from acute infection. In any case, I think that there are good reasons to prefer the scientific, nutritionistic arguments offered to mere paleo reasoning (especially when paleo thinking doesn't return a clear result, as in this case).
And a final addendum is that even if I thought grassfed meat were significantly healthier, I'd still eat conventional beef for the ethical benefits, naming reducing my unnecessary expenditure and increasing the amount I can give to foreign development charities.