The Super-Diplomat ability has sooo many benefits that go beyond simply getting uber techs for scrap. Just keeping yourself out of wars for one is worth the ability. Not to mention easier Economic/Research treaties (which raise the AI's opinion, also keeping you out of wars more easily). Super-Diplo also lets you speak to the AIs more often (fewer weeks between), and even when AT war you can usually still get council with your enemy (when otherwise they'd ignore you).
I definitely give the Terrans a try in my next game. How does this ability fare in 1.5X (not including tech trading) now?
I agree, Terrans still rock. It's still easy to stay out of wars even when your military/weaponry tech lags far behind. It's easy to bribe other races into either liking you or, even better, into warring on their neighbors. I spend extra effort in the early stages of the game making sure everyone's at war (thus expending resources on their military) while I try to get my tech up to speed and develop my economy and infrastructure.
I play miltary campaigns exclusively (for now) so can't comment on the ease of alliance victories. I've won with influence victories before, though, and influence is another area where Terrans can shine.