I have lots of seat time in my 35i with Stage II, never tried an "is" . I figured that having the extra 78 -80 hp all the time more than made up for the lack of silver mirror covers and only 7 seconds of boost.
That said, I'd probably get an "is" today if I didn't already have an "i" and then Dinan it.
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Member ZSCCA 2009 Z4 s35i with Dinan Stage II software, Sport and Exec packages, 19" 296 rims , a stubby and mud flaps
'03 Z4 3.0i Sterling Grey (Hers)
'01 Z3 3.0i Sapphire Black, M sport package (Mine)