I think proprietery parts is only bad if a gun is unrealiable, underperforming and if getting the original replacement parts is hard.
When it comes to pimping out a gun, the good thing here is you are offered guns that are visually and functionnally already equiped with nice rails, a comfortable grips and such.
And as far as internals go, the GT Advanced series already has great performing internals insuring you don't need to mess with anything.
Black Coqs - Fireteam Echo
FAQ Boot Camp Certified