Silencing a straight pipe system ( TD )

Hello guys.
I've got a mk1 1.6 TD, I've done a 2.5" straight pipe exhaust turbo-back few days ago.
It sounds amazing but after few pulls in city traffic its pretty loud and i don't want to lose my nr. plates. Engine woke up greatly, spooling faster and pulling stronger. What should i do to silence it without sacrificing a lot of performance and turbo noise?
Would a free flow back box do the trick?

Yes, but so would a Resonator.

My Brother who rocks big V8's in Corvettes, replaced his exhaust got rid of the Catalytic pervertors (lives in a Emission Free State),  Res and Mufflers (back box).

He had installed some Large Fender washers on a Pivot that are counter weighted, I can ask where he bought them…. but they muffle the car at idle, and fully open under the slimmest throttle….  (works like a Butterfly on the carb).  really quiet on the road at 110-115 MPH, and at idle in traffic you barely hear it…. Until he opens it up as he is want to do from time to time.

A resonator is a Pneumatic Dampener.  It is a Hollow Tube wrapped on a open pipe that stops the Hammering effect of the Exhaust Gases (puffs), to Quiet it down.

It doesn't affect the performance at all.
It stops the Hollow Ping.. I don't know how better to describe it, but it stops the expansion of the pipe by holding the expansion in check.