I have been 'testing' after market gaskets, sets also. In my experience their quality is far worse than the original ones you buy from Yamaha. Especially head and cilinder foor gaskets, and the valve stem seals. They just start leaking after a 5000 km.
What I do nowadays is buy those last items from Yamaha as well as the not-so-expensive-but-difficult-to-make- yourself gaskets as the clutch cover and alternator cover.
The prices for O-rings from Yamaha are outrageous. Just take the old one and go to a good car part shop, and buy ten for half the price that Yamaha charges you for one. Same for shaft seals, except for the one on the outgoing shaft from gearbox, should be original Yam also, if you look at it it has a special lip to keep the oil in.