I was using solely Vpower for the first 12 months, but then I moved and am no where near a shell garage but 2 seconds from a tesco one. Use Tesco super all the time now and tbh there is no difference at all.
VTS could quite easily be making more power with simple mods.
TBH I'd be more upset by the GSI. Would say there is something definately wrong with your car.
Fred @ BTM Performance supplies the YozzaSport exhausts and they are the very best new aftermarket exhaust you can buy.
If you dont mind second hand though, in my byast opinion my Yozza based single exit is better as the flat spot is less noticeable, but all the other performance benefits are...
The ones on the doors have a single screw iirc which you need to remove, open the door and inside you'll see a black rubber grommit, remove this then remove the screw and try not to drop it inside the doors.