Assassins reach x7 at Level 21. I think they hit x6 at Level 17.

At Character Creation, they get a +1 bonus to their THAC0, as compared with other Thieves and Bards. This gives a little help in combat, but their biggest boost is from being in Stealth--I believe all characters in Stealth, Rangers included, get a nice THAC0 bonus for their next attack. I've never seen it in print, though.

Their 15 points per level isn't bad, you just have to work around it. Ignore Pockets, Locks, Illusions, and Setting Traps for now, and concentrate on the most important 3 skills. When you've got them up to snuff, start learning the other skills, one by one. There are also a LOT of items that you can find or buy that will improve 1 or more Thief skills.

as far as thieving skills, my assassin went to TOB with some great skills, they take longer to get but as long as you focus on the key skills such as setting traps and such, youll be ok. plus there are a bunch of items that raise your skills...mercykiller ring, and the leather armor that upps your hide and move silent.

That is correct - when attacking from the shadows, the character recives a +4 attack bonus.