Personally, I think IDC is so generic, that it doesn't really matter where you take it. The Course Director does matter - as they define the quality of output. Most just prep candidates to pass the I.E., rather than create quality instructors.
Matt Bolton at Crystal Divers did a good job with my IDC. I was already a BSAC instructor, but he still managed to make the training beneficial and interesting.
I've not been on Koh Tao since 2007. Back then, Master Divers/The Trident/Jamie McCloud was THE place to do tech training. Other centers have subsequently opened tech training outlets. Best advice is to visit in person and inspect them.
Alternatively, consider IDC on Koh Tao, then look further afield for technical training. Does it have to be PADI? (intentions to progress to TecRec tech instructor?). If not, then definitely consider
Bruce Konefe in Pattaya, who runs ANDI courses at every level (up to instructor-trainer), including multiple CCR units, cave explorer (level 5), advanced overhead sidemount.. and even a 180m Trimix qual... Very exceptional training indeed...