Any "Caddy PCM" in the year range, with wiring that would mate and deal with all the same functions would be "Locked" as well. It isnt "Locked" as I hear so often. It is "Non-Flashable" It isnt so much they wanted to keep us out of it, as it was conceived and built to work in a peculiar manner that doesn't allow for alteration and still do the tasks assigned too it.
Chuckling,... Your beating a dead horse... I am going to get long winded here and point out some very hard facts. And save it for the next enthusiatic new GXP owner in who'* steps I already walked. Several things make appreciable mods "Not gonna happen". First the electronics/programming of this engine are complex. Limp mode, variable ignition timing, self diagnostics, an aggressive variable ignition curve/ fuel~air ratio, etc. And a lot of it runs dependent on the info from the balance. The PCM even has a "learn procedure" that logs internal readings and takes them into account as it controls the functions. GM/Northstar doesnt want people messing with this and took steps to ensure that. It isnt a matter of, ..."if someone could crack the PCM, we could alter things". The fact is the PCM isnt "locked" it'* outright non-flashable.
Even if it were (and its not) The LD8 is a rare limited number engine in the aftermarket parts scheme of things. In it'* present form, it is only found in the 04 and 05 GXP'* and the Buick Lucerne (06 on, as an option). There were only 2400 GXP'* made in 04 and far fewer in 05. With the small percentage of engine owners who modify them, specific aftermarkets parts arent finacially feasible for manufacturers.
After almost 3 years of looking myself and pouring over others search'*/discussions, there are only 3 possibilities for a real horsepower gain.
1)Nitrous (75 HP shot works well)
2) A rear mounted turbo ( You would need the universal application model for this Mod) I have heard a few spout off that this is what they are going to do and have a friend to help out (walks away snickering) For this mod you need, a) $2995/plus piping and the money for someone who can do welding/fabricating to this degree.
And after all that you have achieved only 6-8 Lbs of boost and no way of knowing how the PCM will register and react to it. While we are in the realm of "The Millionare Boys Club", hey...lets add another turbo.
. Sorry, I couldnt help myself.
3) Replace the PCM and have someone with the sophistication and experience/knowledge to tune a Northstar engine. A few are avalailable $1800 up and a few places that can do the tuning. RSM Racing, Mallet Cars, and CHRFAB/California. You will need a second mortgage on your house to accomplish this one as well. And thats just a starting point for other mods to be considered.
This dead horse has been beat enough. Enjoy the car for the extra performance it has, for a 2 ton Family Sedan. And it'* refined, uniquely styled, appereance.