^^^ Agreed. I wouldn't go more than GSC 268's on a stock head even though some people have had luck with 272's on a stock head. I'd go with the Titan 264's if I were you If anything were to happen, you have the support here in the US whereas HKS is shutting there doors here in the USA and frankly, their customer support isn't really all that anyway....
love my gsc s2's on ported head +1 valves and 70.3mm turbo
only lost 200rpm spool off stock cams and added 2000 rpm to powerband (idle is pretty wack though compared to stock. but that is the best part)
i cant imagine gsc s1's i practically gaurantee they will be ultra.
if i ev er have a smaller turbo i will run gsc s1 no questions asked.
i believe they are 269 duration and not sure lift. 9.something milimeters.
might make your motor interference but it will be worth it.
the gsc s2 are 274 duration 10.2mm lift and way too much cam for a 61mm turbo.
gsc recommends S1 up to 71mm turbo and S2 71+mm. but there is a gray area depending on some things (as with anything)