50% more for a 2 person alliance. (If you're starting the game with teams of 3+, add 50% for each additional person.)
Unless you're signing a PA with a very weak AI (who generates less than half your breakeven beakers per turn), your overall tech rate will increase in the PA. Signing a PA with a stronger civ is of course more beneficial, although you may have a harder time convincing them to go into it.
Even signing a PA with a weaker civ can be good if it'll net you a bunch of free techs. Don't forget the instant tech transfers are part of the benefit of PA's. If the weak civ you sign a PA dies, it should still be possible to sign with someone else later on.
As I've mentioned, tech speed shouldn't be an issue. However, bear in mind that in a PA, if one of you is building a wonder then it disables the other from building it. So if you're already building those wonders you should be good. You can also queue up future wonders so your teammate can't prevent you from building them. The only thing you have to look out for is if he starts building a wonder immediately on the turn you research the tech for it. Nothing you can do in that case, unfortunately.