So far so good.
I'm not sure where the spark should be - part of it is how much turbulence you have and that is partially quench. Did you measure piston to head clearance @ TDC?
If your cranking compression is high (above 170?) the cam may be advanced beyond the mfg. suggestion, and retarding it 2-4° will pick up the top end in general.
Have you eliminated the obvious: are both throttles actually vertical when the linkage bottoms out?
The air filters look big enough not to be the problem.
Have you tried backing off to 7/8 throttle - is it better or worse?
How much valve spring tension?
