I'm still running the stock one on my shee. But I've heard, Pro Design, Mull, Chariot,..those ones seem to be nice from what Ive heard.
I know Chariot has two different impellers they offer, A standard one and a high flow one...maybe look into that.
I run a 16PSI Mr. Gasket radiator cap with the built in temp gauge on mine.....seems to work well. Maybe try that. I know there is a topic on here with the part number for that cap.
Congrats on the banshee purchase!
With 140 you should have no problem running 91, I think cut off is like 165-170. Here is a good topic to read on compression and race fuel: http://bansheehq.com/forums/index.php?showtopic=116369
On the gear oil I just use cheap ATF type F, and on the coolant just normal 50/50 coolant. I will probably change to engine ice coolant next summer. Everyone will have a different preference on gear oil, coolant, oil...etc.