Clipless pedals make a huge difference.
I can't put a mph average on it. It will make the biggest difference on hills. I wouldn't be surprised if you can add 1-2mph to your average. Definitely go clipless. Don't look back.
I have only done two tris. Both with tri shorts. I found them to be comfy for all three legs. Its a bit quicker not to have to throw runnings shorts on over them. I think its worth the $50 or so.