stage 3 and 3.5 have no english translation 
Phase 3 (at date I had this phase 3 on my car) 284 PS - 457 N.m :
Precat removal, installation of by-pass
Balancing shaft removal
Large intercooler installation
Airbox kit installation
Larger fuel injectors installation
Z20LEH turbocharger installation
Additional oil cooler installation
ECU reprogrammation with phase 3 soft
Phase 3 (In a next futur, I will have this phase 3.5 on my car

) 309 PS - 494 N.m :
In addition to phase 3 phase 3.5 include :
Z20LEH turbocharger with Dbilbas manifolf
70 mm exhaust line
ECU reprogrammation with phase 3.5 soft
It is important to do not confuse this upgrade, with other german upgrades like EDS propose (or REGAL in UK)
IMO REGELIN ECU softwares are better designed and well suitable for our VXT (REGELIN's cars are VXT).
Tests I had performed before, in order to compare my former EDS phase 3 and my new REGELIN phase 3 shown signifiant differences in runs results and better behaviour of engine.