Is switching from a PSI harness and stock ECU to a Holley Terminator Max worth it?

It depends on how far the build goes.

Stay with the factory ECU + standalone harness if: the engine is close to stock, you're running a factory-supported transmission, and you're happy tuning with HP Tuners or EFILive. It's the cheaper route — the factory computer often comes with the junkyard engine — but Every donor PCM needs reprogramming: delete VATS (or the engine will not start), match injector size and MAF scaling, and match transmission settings; emissions monitors and speed limiters from the donor are also present.

Move to a Terminator X Max if:

• the engine is heavily modified or boosted — LSX Magazine considers the self-learning Terminator X better suited than a GM ECU there;
• you want engine, drive-by-wire and transmission control in one box: the Max has 4L60E/4L80E control built in, and 6L80/6L90 control with Holley's 558-499 kit;
• you want Holley's included engine and transmission harnesses instead of a separate harness.

Price check: Terminator X Max kits in our catalog are listed at $85.79, $250, $375, $617 depending on engine and transmission, versus about $599–$769 for a PSI standalone harness to keep your factory ECU.

More in tuning an LS swap and stock PCM vs Terminator X.