Would this combo work?

I have a 551 EOTech forward mounted on the lower rail portion so it can co-witness correctly, but I find the front sight tower a little distracting (fine with the Aimpoint, but the reticle size of the EOTech floods the tower).

Maybe just get another PRI?

When mounted on RAS it should be the same height as the barrel mounted sight.

Is the lower level of the ARMS #50 on the same plane as the Knight's FF Ras (or more correctly the same plane as the flattop receiver)?  That is probably the best way to look at it.

TIA

The bottom rail on the SIR 50 is the same level as the flat top rail so it should be the same as a the KAC rails.

I thought this would be a problem too but when I use my 552 on my SIR 50 the front sight just blurrs out.  When I have both eyes open and use a hard target focus I don't even see the front sight.  

If I close one eye or focus on the reticle then I can see the front sight.  The EOTech was designed to be used with a traget focus so you may just need to do more training to get used to it.

As you mentioned the other alternative is to get a flip-up front sight.  I'm not sure how well a Knights flip-up sight will work on a SIR.  I'm not sure but it looks like it may not have enough room to flip down with the EOTech mounted.  I would probably go with a PRI instead. I'd give it a few more training sessions before spending the money on a PRI.  Just my 2 cents![:D]

I am planning on purchasing a #50 SIR and I currently have a Eotech 511, could you show some pics of your setup please!
Thanks