I've not tried the Lapua, but I have run a few boxes of Fiocchi through my 59/66.  It is great ammo in my experience.  It grouped well and I had no slam fire issues.  I just can't justify the cost over mil-surp or Wolf right now, though I have been collecting the brass when ever I see it.

Just curious on the Lapua ammo, as about half the AK's accuracy "problem" seems to be the lack of quality ammo.

I've tried Lapua FMJ ammunition in my Arsenal SAR-M1. Accuracy at 100 meters appears to be about 3MOA - the same as just about any western (including Sellier & Bellot) ammo. (On the other hand, the Wolf ammo I've tried has barely kept all hits on an IPSC target...) The only thing it seems to do better than most ammo is that is has a very small muzzle flash in low light conditions - but of course, I haven't been able to test this extensively.

For best accuracy, I prefer to use Sako ammunition in my AK. It's quite a bit cheaper per round than Lapua and 100m groups sizes can be 2MOA. (From a bench, with a scope.) The only downside is that it only comes in 250 round boxes, which might make testing this ammo a tad expensive proposition for someone not quite sure of it. I am also unsure whether they sell this ammunition at all in the U.S.