I've honestly havent spent much time there at all. I know you'll get convoys that go through the straights of gibralter along with task force groups, but overall i never had much luck with merchant traffic. Again, havent spent much time there. I might give it a go after GWX is released for something new.
As for an IX boat in the med, generally speaking, YES, wrong boat for that theater. The extra fuel is unncessary, and given the air cover, a faster dive time is MUCH more desierable. That said, that hasnt stopped me from bringing an IX boat into the med.. infact.. i didnt just bring one through gribralter, i brought one into the fricken black sea, just to say i did it.
Hmm, come to look at this one, i did leave a trail of sunk ships in my wake.
And yup, i brought the boat back out to the altantic as well. One thing ive never tried was the suez canal.