There have been a few posts about this before and is a common question really, I dont think anyone has done specific in depth testing, although Brian Samson will have done some with different batches and knows a lot more than most of us on BC calcs especially down range.
I tried 7.9's in my MPR's and EV2 in comps, and found that although they were grouping better indoors or still conditions at distance, take them into the wind and they are not as predictable. I found that generally 8.4's follow an expected path and follow the wind, the 7.9's spiralled any where and were very unpredictable. Not all the time but enough to knock your scores badly.
I still think that being slightly shorter will not help 7.9's to stay on track with a side wind, not just the weight.(I cannot prove this of course, it just my theory).
Then others say they are great so you will have to test them in your own gun, and see how predictable they are. They do usually strike a fraction higher at distance, but not enough to gain much of an advantage.
I would rather now have a predictable 8.4 pellet that has a consistent group without fliers than a super tight group that chucks the odd flier or is unpredictable.
One thing is for sure, if you find a great batch go back to the shop as quickly as possible and buy all you can afford.
I have just bought 7 sleeves of a batch I found to shoot well in my MPR's, luckily my Steyr that I have just switched to likes them as well.