TBH, unless you are running a tighter and longer bore than 6.03mm x 455mm I wouldn't give much of a sh!t. In my ICS AKS-74U Blaster 0.25's & 0.2's are no better than BB King which are mega cheap, yet still highly polished (you can see it when you get a magnifying glass on 'em next to Blasters), as perfectly spherical as any more expensive BB, and 5.95mm diameter. The difference is slightly noticeable in my CYMA tac AK with 455mm x 6.04mm barrel - in that I get more 'fliers' with the cheaper BB King's and maybe 2m max extra range with the Blasters.
In my Ares L1A1 which has about 550mm of unknown diameter barrel the difference between Blaster 0.2's & 0.25's is generally more noticeable than the difference between 0.2's made by Blaster and BB King in a flatter trajectory and better accuracy, but the range difference is more noticeable too, at about 3.5-5m more out of Blasters than BB Kings.
I haven't fired enough BB's through my modded G36KV with 509mm TK hop twist barrel to form much of an opinion, but so far the difference has not stood out enough to make me think "Right, I need to only use these BB's."
I've also got an unmarked bag of about 2000 white 0.2's which are suspected to be Excel. I've used them around the house to test fire and out of my back window - they seem ok.
I must mention that I recently installed a 6.02mm barrel in my AKS-74U and decided that, unless push came to shove, I would stick to ammo with a better rep. I bought 9000 Blaster 0.2's on offer from Patrol Base for £17.48 + P&P (but I needed some Krylon anyway so P&P was shared). I buy BB King's in bulk from a bloke on ebay.
Oh yeah - King Arms 0.3's in my SVD but i've been advised to try Madbull 0.36's, which i intend to do. I use Blaster 0.28 green "Stealth" BB's (ebay) in my pistol which probably don't give as good range as 0.2's because it has a fixed hop, but i don't care b/c it's only for desperation stakes and I want ppl to feel those bastards hit them!
I think my main point is that cheap BB's have not been sufficiently bad for me to think it's worth paying almost twice as much for BB's with a better rep. Especially for days when I'm going to fire a shed load in a supporting role, like 5000 or something.