Just bought the FN PS90. It has the USG sight - hope to shoot it this weekend.
Size comparison -- I was shocked to see how short it is compared to the MP5! I do plan to SBR the weapon, so it will be even more compact.
http://www.mustangmods.com/ims/u/293/327/187722.jpg
http://www.mustangmods.com/ims/u/293/327/187723.jpg