The upgrade to the TSII isn't so hard and doesn't have to be too expensive. This thread
has some good comments on various folks ideas on upgrades. I described my upgrade process on my hobby web site here
. You can get the extra wire to the nacelles without major surgery. You can get some nice motors and ESCs from Hobby Central for a good price.
I know you mentioned that you were thinking about a mid or low wing, but you also said you were looking for a big foam second plane. I enjoy flying my Magister. It is being replaced with the Mentor which is the same size but a tail dragger. It's a big foamy and strong. I nosed mine in from about 200 ft when the ESC blew up and was shocked at how little damage there was.
A balsa plane would have been kindling. Here is a link to my Magister
write-up. The Magister with the CAP paint job is post crash. With a nice brushless motor, you'll be able to do the basic aerobatics you mentioned in your post. It is no Acromaster but is a great large second plane.