To put this up at the beginning, sorry if this has been mentioned before!
It has came across my mind several times since I started playing that using your gear doesn't really seem to affect how fast an item lvl's up? Am I wrong here or is this actually the case?
It doesn't, no. As you progress through a level you gather heat (the little glowing orange balls) and when you get to the lift at the end, it is distributed evenly to the equipment you are wearing.
In short, you don't have to actually use items to level them up.