A dump valve will release your inlet charge between gear changes and stop the backpressure from stalling the turbo (and sometimes shattering the impellor fins in the turbo) it will also help to sustain the turbo spinning, and deliver the power quicker after a gearchange.
I'd recommend an exhaust & air-filter as the 1st mods to do.
Then a bigger turbo & decent aftermarket boost guage, intercooler, bleed valve, water injection... the list goes on