The 98 GT only put out 225 HP stock your Bullitt is putting out some where around 270 HP. That is a 45 HP diff, and to gain that much from a motor you have to put the type of mods on that your buddy did. If you look at the April issue of MM&FF you will find an article where they took a 99 Cobra and a 2000 GT and put the following mods on the cars: (All BBk)long tube headers with high flow H-pipes, CAI, under drive pulleys, and a larger throttle body. After those mods they both only increased 17.0 - 17.5 HP. If tuned they might get more. My point is that his heads is the only other mod and that accounts for the other 28 HP. I would say he should be slightly higher in HP but he may need to dyno tune to get it.
Not to mention how the power is put to the ground through gearing and such makes a big diff. So, you can see that Ford has put together a nice package with the Bullit.
It also goes to show you that it takes well matched mods and a good tune to really get the gains most would like to see. That is why It is almost futile to change much as far as intake and exhaust go on the Bullitt. I believe that bettering what is already there will get you close to 300 HP. Extrude hone intake, headers, CAI and a good tune just might get a Bullitt up there. IMHO