I assume since you mentioned iron sights on your p90 that you have the RDS version, I suggest (although a more expensive route

you get a DTP rail and add whatever optics you like including the square RDS thing that you have up there.
The RDS receiver cannot use any aftermarket optics such as the one in your link. If you have the TR version then it'll be peachy. But considering the TR doesnt have Iron sights... well.
Here's my setup with an extra rail and an aimpoint, I believe it elevates it even more than the TR so it's extremely easy to sight even with a full PB mask. (i wear full mask indoors)
http://i3.photobucket.com/albums/y82/valcrow/p90_03.jpg
and here's the rail:
http://www.uncompany.com/pageproduct...asp?prodid=718
gluck. p90s rock!