New 2011 GTI, whats a good sound system?
The Mustang might have had the 'monsoon' system, which is 500-1000watts depending on the option with a sub and proper amp. VW has decent systems for what they are, but comparing a 250watt to a 500/1000watt isn't the way to do it.
You're best bet is to check out a local sound shop and see what can be done. Keep in mind VW can cancel out on any comprehensive warranty repairs as they can say the bass of the new sound system was the cause of the problem... just FYI. I dealt with that problem with my '06 G6 GT. Apparently, the 12 disc CD changer I had installed in the trunk at a shop was the cause for my loose front speaker screws! Took GM 3 days and a piece of my right fist for them to install the proper screws.