Yup there's definitely some added weight on the front; I added a Nitro.vo sleeve to mine, but ideally you'd probably want a metal body. I also switched to a Crane stock, since mounting a large battery in an AN/PEQ to an already heavy front end would have made the gun way too front heavy.
The realism part I can deal with, honestly I find the airsoft RIS-mounted M203 more "realistic" looking than the XM-8 or FN2000 :P
One of the reasons I opeted for RIS though is I'm using a Commando -length (11.5") barrel setup, which wouldnt accomodate the regular mout properly. It also allows me to mount or remove the M203 in 5 seconds flat, and lets me mount it on any RIS-equipped gun without any hassles. For practicality's sake, I was willing to forego a slight bit of realism.
And as Obelisk observed, there is a slightly larger gap, maybe ~1cm. It's not that noticeable unless you know.
"Proper" mount:
http://www.ussocom.net/images/products/4_zoom.jpg
my setup:
http://tacticalmoose.com/pic/933RAS_proto2.jpg