the northstar engine thats in the Eldorado ETC and ESC are both the same. However the horsepower difference is from the different camshafts used in the engines. The ETC which has the L37 has better intake cams which give it better horsepower (25hp more) while the ESC with the LD8 has better exhaust cams which gives it a better torque curve (5ft/lbs more). The Northstar engine has 2 engine codes to identify which engine has what power and what vehicle it is used in.
(VIN "9") high-output 300 hp @ 6000rpm and 295 ft·lbf @ 4400rpm
Eldo. ETC, Deville DTS, Seville STS
(VIN "Y") high-torque 275 hp @ 5600rpm and 300 ft·lbf @ 4000rpm
Eldo.ESC,Deville DHS, Seville SLS, Deville Limo, and hearse
I didnt include all the vehicles but for the most part you get the idea. But if you still want them extra 25 ponies and dont want to spend loads of money then get an exhaust and intake system put on to help it breathe better. Although you will get more power by doing this you may not like the out come as far as the louder exhaust and louder intake noise. i have an intake system on my Seville and while it sounds mean when i pass a car on the highway it takes away from what cadillacs are meant for. but by no means am i saying not to do this it your car and your choice as to what you want to do. I just wanted to give you more insight on the diferences. now that im done babbbling ill let you get back to what you were doing.