does anyone have a pic of their a2 with a SIR 51.
does the fixed carry handle model work as well as the flat top

Only one I actually saw was at MSP's booth during the Shot Show. I cant say I know for sure they are as solid, but, knowing how the flat top SIRs mount, I would imagine the #51 is probably very close to being as sturdy if not as sturdy.

Instead of being mounted securely by three cross bolts to the upper, and with the locking yolk around the barrel nut (secured by four more bolts; two on either side)... The #51 would be atatched to the carry handle both through the hole in the center channel, and by a locking bar around the front of the handle. This is in addition to the same 4 screw locking yolk around the barrel nut.

Even if one may be stronger than the other (and Im not saying I think one is), but I would imagine either are probably stronger than anyone could ever hope for. I am not, however, an engineer and am really not terribly qualified to make these statements as fact. I do think its a pretty good guess though [;)]

Though I haven't seen one myself I would say it's a darn good guess!!!  

I would think the only difference you could find would involve stress measuring instruments most of us don't have access to.

I have one (#51), I also have #45. IMHO, #51 is as strong as #45. It looks great. The good thing is with #51, you don't need a backup sight #40. You don't need to worry about the height of the front post sight provided you have this kind of problem.

If you are familiar with A2 sight and can use it effectively, then it is a good platform for you.