I appreciate the kind words, but I'm just more of an enthusiast. Luckily, over on the Allante board, we have a GM Engineer who hangs out. He's been a wealth of info.
The big thing is getting over the ECM. Allantes (and, I think most Cadillacs with 4.x engines) use a speed-density fuel management system. It has a crank sensor in the distributor, air pressure and temperature sensors, and throttle position sensor. Essentially, because the volume of the engine is fixed, the ECM uses that, and calculates the density of the air based on the temperature and pressure. With the air density, and volume (engine displacement), it knows how much fuel to add.
Obviously, an easy way to get more HP is to boost the displacement. Since there is parts interchangablity between 4.1/4.5/4.9 (Cadillac first bored out the 4.1, then stroked the 4.5) its relatively easy - except for the ECM which relies on the displacement in its fuel calculations. There are ways around this, but none as good as reprogramming the ECM.
Because Cadillac used a bunch of ECMS (the Allante's are different than the other models, plus other variations) and because Cadillac ECMs incorporate the onboard diagnostics capabilities, no one's invested the time to hack a Cadillac ECM (at least not that I'm aware of). If this hurdle were jumped, it would open up a small, but significant, performance opportunity. One other issue - the Northstar Cadillacs ECMs became PCMs (Powertrain as opposed to Engine Control Modules).
Another alternative would be to use another GM ECM. For me, the onboard diagnostics isn't worth giving up, especially as it would be crucial to tune the new configuration, IMHO.
Finally, you have to worry about the transaxle that the 4.x engine is attached to. Allantes have the F7, which, I believe, is a modified 440t4 (4t40). Later Cadillacs had the 4t60e/4t65e where the PCM adjusts engine power so that the transaxle doesn't grenade. Northstars have the hefty 4t80e, but even then, engine power is modulated to improve transaxle life.
Hope I'm not discouraging, but these are some of the real issues that unfortunately have made this great engine sort of fade off into the past. It's never going to make 500hp, but it definitely has some potential, if you could get around these issues.