I believe the only way to change the attributes of a different weapon is to alter the mods that are on it (+25% against synthetics, better sights for +9% acc). As an infilitrator (I'm one too), you're limited to the pistol and sniper rifle, as you can't spend level up points on shotguns/assault rifles - or 'aim/zoom in' with rightclick.
To improve accuracy/damage, you can spend those points on pistol/sniper rifle. I find that by always having a decent pistol, it serves me quite well in this game. I do admit that sniping can be a pain.
Neat sniping trick: If you're taking cover, try to line up your shot carefully before you zoom in, if you're on target, the reticule will go orange, and you can rightclick to pop out and cap em instantly.
If you can't get it perfect, I find the 'wobble' gets worse with time, so lining up, even vaguely and taking a shot before popping into cover seems a better option than staying zoomed in trying to control the wobble for too long, and leaves you less exposed.
EDIT: I just take 2 folks with assault rifles with me, and use marksmen on the pistol near-constantly. I do miss when sniping occassionally, but then I should spend more than 3 points on it - just so much else to advance in!
Edited by AntiTalent - 6/13/08 at 1:24am