You're not removing the cat so no spoof is needed. I don't know why you would need a 3" downpipe, but it would probably free up a few horsepower, as long as you leave the rest of the exhaust stock behind it. If you open up the whole exhaust to 3" you will lose WAY too much backpressure.
If that doesn't convince you maybe this will:
My dads GTP (the one in my sig) runs 12.2'* @ 115mph...it has a 2.25" borla exhaust on it. Yeah, it has headers and a 3" downpipe, but the downpipe then reduces from 3" to 2.5", which then reduces to 2.25" where the exhaust bolts on. Having the big downpipe but keeping the small exhaust frees up any restrictions before the cat-back to give as much power as possible, but keeps the backpressure higher due to the smaller exhaust. The GTP pulled 315.6whp with the 2.25" exhaust on. I can pretty much guaruntee you that if it had a 3" exhuast it would have less power. With open downpipe it pulled 304.9whp. So basically, don't go too big, afterall, it is still a 6 cylinder.
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!