You don't get any extra perks or anything like that, it just increases the attack value of the weapon you are upgrading. So let's say I have a Suros Regime at 300 attack and I infuse it with another Suros Regime with the same attack value. All of the perks and stats (Reload Speed, Magazine Size, Stability, Rate of Fire, Impact, and Range) stay the same, but the attack value will rise to 310 (or something higher, I'm not quite sure on how much the attack value will rise, only that it will increase upon infusing a weapon).