I think you need to be looking into a good pre-muffler or resonator... not a bigger rear muffler.
IMHO the borla with the wide-spaced tips looks pretty poor, little too rigged, little to un-supra like =)
If you wanted a custom job and not a new catback, i'd look at Tanabe Hyper Medalion mufflers or something like the Greddy SP2. Both very large oval cans, top-notch quality and good looks. Combine those with a Magnaflow/dynamax/borla/whatever 3"in/out round resonator/premuffler under the car and I'd think you'd be set.
If you don't want to spend that kind of money, there are decent knock-offs of each on ebay =)
I always thought Megan made pretty decent stuff for the price... if you like its looks, I'd say just weld in a pre-muff and keep it.