Efini-Boy,
With dump to atmosphere on a MAF driven system (Standard System) the MAF calculates the amount of air required pre dump valve, so if this air is dumped, yes the car will run rich on initial throttle open. Over fueling. This in turn can cause stumbles in throttle responce and even eventual damage to your lambda sensor that is standard on your ECU.
If you want a Dump valve to assist in turbo lag reduction, you will need a nice re circulating sequencial bov like the HKS knock off available at Turbo Toys, which comes with re circulation adapter. But in this event, u must not be after the sound.....
If you are after the dumping sound, then you would require an ECU with a MAP sensor, wich as the name sais, is a Manifold Absolute Pressure sensor, this then measures the air pressure directly in your manifold which is not affected by a BOV or Dump Valve. Then u would be able to use BOV's like the Tial 50mm range even.
Best thing anyway I feel is to go After Market ECU, but then either have it installed one time shoe shine by a tuner that is accredited with that system, or be willing to have ur car stand for ages while u try figure it out urself.
Hope this gives you better insight.