I'm not too sure on the 98 3.8l V6 but if I remember correctly, they made about 150 hp and like 220 torque. They also had a 0 to 60 of about 9 seconds, depending on wheather you have a 5 speed or auto. Is yours a 5 speed or auto?
The 04 Jetta's had the 1.8l turbo I beleive. And they made about 180 hp and 174 torque, with a 0 to 60 in the high 6's.
So basically, if he is all stock, and you have your mods you listed..I think it would come down to the driver honestly. You already have him on torque giving you the jump if you know how to drive, but then again, he probably weighs a little less than you so it mite even out. But if you want to get a better hp number and have a really good chance, get an intake and a throttle body spacer with your current mod list. You can find them on ebay dirt cheap and they are very easy to install. Atleast with these, you can get a little closer to his hp range and have a better chance.
Here are some links:
1998 V6 Mustang
94-98 Mustang 3.8 V6 Cold Air Intake+Filter Polish 3.5" | eBay
1998 V6 Mustang Throttle Body Spacer
96-98 V-6 3.8L FORD MUSTANG HELIX THROTTLE BODY SPACER | eBay
I hope this helps man