Porthole do you step on your floorboard to climb on and off the bike??
I tend to stand up on the right board and then put my left foot over the seat to get on and was wondering if the extra long bolts and the leverage put on them would be strong enough to hold me up (225 lb...OK 240....alright already 250 lb) 
Maybe grade 8 bolts instead of stainless would add some strength.
Any noticeable floorboard scraping on the twisties.
My guess is that allen heads are used because of the limited room at the mounting locations.
Spacer part numbers at McMaster Carr
2" 92510A832
1.5" 92510A829
1" 92510A826
1/2" 316 stainless Allen head screws
2.5" 92185A722
3" 92185A724
Rockwell Hardness B70
Minimum Tensile Strength 70,000 psi
Thread Fit Class 3A
The screws are 1/2", fairly hefty.
I do not stand on the floorboard to get on and off.
I did a little more scraping after extending the boards, but you can compensate for that somewhat with the way you ride. And I have since raised my bike to "normal" height and I no longer scrape.
The increased comfort far outweighs the occasional scrape in my opinion.