Forge use to make and upgraded diverter/recirc valve - think it was about £300 fitted??
Dump valve to atmosphere = bad idea for a diesel (turbo stall!)
Diverter/ recirc valve = no benefit £300 worse off.
You want a tdi to chirp, cut the airbox & a performance filter. Then a cat bypass and a straight through exhaust system!!
My old Fabia VRS sounded amazing