Real steel P90's dont have a laser like that stock, I'm a P90 fan and have never heard of that before except as an aftermarket addon.
As for taking it to the limit, I dislike the looks of guns with everything but the kitchen sink on em, I actually think they are ugly and ridiculous. I stick with plain and simple add ons, when I get my P90 (no idea when, may be more than a year) I'm getting a reddot sight, sling, and thats it. Currently my M733 has a reddot and front vert grip, and my CA MC51 when it comes in is getting a reddot sight and MAYBE a vert grip.
The thing I hate the most to see on a P90 are those damn M4/16 hicap mods, it just looks plain stupid, just get a damn CQBR if you want to use M4/16 hicaps. I love the look of the P90 in its externally stock form with nothing more than a reddot sight and maybe a taclight, nothing more.
You want to deck it out and stay reasonable? Its a TR so it has the tri rails so scope/sight on the top, tac light on one side, laser sight on the other, and either the two point slings that need the front over-the-barrel bit and the rear bit, or one of the slings that just goes around the body. I pretty much just described the P90's on Stargate SG-1 (I was a P90 fan well before they used them on that show) so if you want to see an example search for some screen shots from one of the mid to later seasons.
*EDIT*
Stargate P90:
