The trick with infiltrator is knowing when to stand still. I think you're 100% invisible that way. I've spent decent amounts of time lurking around inside moderately populated enemy bases just sabotaging stuff without getting caught. The motion detectors actually help. Place them at entrances. When something triggers them, you know someone is coming and you need to get yourself into a corner, cloak when you hear approaching footsteps, and stand perfectly still as they pass by. They may guess at where you are if they have any clue to go by, but I promise they won't see you. Cloaking is also a good way to move across open ground IF you haven't already drawn attention to your position and you don't pass close to hostiles. It's very rare for people to notice you when cloaked at a distance if they're not looking for you.
I do think the infiltrators need to be more silent. Knife kills are way too difficult to get. If you try to kill someone with just the knife, they will almost always spin around and shoot you in the face before you can stab them twice. You need to stab and then follow up immediately with two pistol shots (default VS pistol), but that gives you away. I think people can also hear your cloaking noise, which is a problem. I understand it's a tough class to balance. People need an opportunity to respond to the presence of a spy or they become the No-One-Is-Allowed-To-Have-Fun class. I think a longer cloak time would help. That way, it's difficult to use cloak as an escape from nearby enemies (can't be surprised by presence of hostiles and cloak when they're already close to you - that's sloppy work on your part), but you can easily use it to get into dangerous areas with some planning.