I wasn't going to reply to this, mostly because I have exams at school this week and I don't have time, but I recieved a PM requesting nicely for me to give my thoughts, so here are my thoughts.
I don't know if you want power estimates without nitrous, or with nitrous, so I will just give you my estimates without nitrous and you can add it in since that should be a fairly simple thing to do. I do not feel that I am familiar enough with the Series I L67 to be confident on my estimates. I am going to assume that it is just like a Series II, only with a blower that doesn't move quite as much air.
Going through all of your mods, I would have to guess that without the nitrous it is making right around 310-320 horsepower and 360 ft.lbs torque at the crank. I am sure that with some work here and there (especially ported exhaust manifolds or headers) and some tuning you could probably get it up to 350hp or so, but because of the small supercharger, and the pushrod 3.8L, it would be difficult to make much more power than that because the blower and engine combination just simply wouldn't flow enough air for it. That is my opinion on it.
1990 Maroon Grand Prix STE McLaren 3.1 Intercooled Turbo, 14.695 @ 94.49 on 12psi before cam/heads/etc.
1990 Bright Red Grand Prix McLaren 3.1 Intercooled Turbo, 14.819 @ 92.72, 189whp
1999 Grand Prix GT 9.75:1|K&N|160 tstat|No Ubend/Cat/Res|UD Pulleys|Ported M90/TB/Heads|XPZ|2.55"|Pacesetters|Stage 2 I/C. 12.442 @ 112.78
2000 Blue Bonneville SSEi Supercharged Gutted Airbox|K&N|3.4"|1.9'*|WP Pulley|Custom DHP|180*stat|41932'*|No Cat. 14.681 @ 94.80
2002 Dark Cherry Grand Prix GTP Supercharged & Intercooled
12.084 @ 114.38 w/1.743 60', 315.6whp, Fastest Stock Cam M90 GTP!
NOW XP CAMMED, 340.6whp with XP, stock heads, Gen 3 M90, 11.984 @ 115.84 w/1.750 60'!
Most HP Stock Heads Gen 3 M90 GTP!
2004 Red Monte SS Supercharged K&N|3.4 Pulley|1.9'*|180*stat|41919 plugs|3" dp|No cat/ubend|DHP|Shift Kit. 14.2 @ 98
GOT BOOST??? Friends dont let friends drive stock!