With bumped up fuel pressure, 42s have done 300whp on a zetec with a return fuel system. Personally I wouldn't try to push that much with a S60, but 250-260whp should be doable. Just make sure to run rich under boost, no leaner than 11.5:1. Make sure the safeguard is on. Don't push much past 13-14psi and it should be good. Probably get an new cam for the intake side and a new manifold like you're gonna.

Yeah, you're not gonna see huge power without a huge turbo(huge in cost also), but the turbo you got with proper tuning should leave you satisfied. 250whp would be almost 300 crank HP and that would be pretty cool also.

2000 Rio Red I4 Cougar 200whp & 210wtq at 9psi 254whp & 276wtq with NX 35shot WRX TD04 Turbo, Cut Short Shifter, Strut Tower Bar, 17" ZN Wheels, Roush Springs, Starion Intercooler, NX 35 shot, HKS SS BOV, Full 3" exhaust,StreetFlight Chip