Before looking to change the chip / eprom, a look at the mapping via ConZult would be interesting, also popping on overrun is symptamictic of other issues, air been drawn into the exhaust, or leaking injector, not seen any generic chips that would cause that as on over runn and throttle closed the injectors and fuel is switched off in line with most every modern ( ish) injection system.
Check cambelt / valve timing, pulsing from valve overlap may have an impact, or even the exhaust type, as said on overrun the fuel should be switching off so check the closed throttle sensor is actually activating correctly.
Generic chips are fine for stock cars with mild boost increase and will perform very well even with larger injectors and bigger turbo`s, only after this stage does specific mapping come into its own, and then its really just fractional changes, by far the biggest way to get your engine running best is to get is set up correctly, ignition timing, cambelt / timing, boost leak checks, recirc valves check for leaks, clean air filters, good afm readings, throttle balance, vacuum pipes checked for correct connections, correct spark plugs, correct oil and good loom connections.
Unless your confident to check all of these and have the correct equipment then get it checked over by Jim at PowerZed you will be amazed how different the car will feel after the essential checks and adjustments are done.