Personally I am not convinced multi probe EGT's are worth the money on a simple carb equiped 4 cylinder. The mixture distribution between the cylinder
is quite uneven, making it difficult to finesse the leaning procedure. My experience with leaning a PA 28 -180 first by the installed sophisticated multi probe EGT and then repeating the exercise by just leaning untill the engine ran rough and then enrichening to smooth operation got me to the same mixture knob position.
A different story for fuel injected engines, especially the bigger 6 cylinder motors. Here a multiprobe EGT is definitely necessary to properly lean the engine
If you want to spend money then IMO spend it on an electronic fuel flow instrument. properly set up these can be amazingly accurate and compensate for the utterly useless fuel quantity guages fitted in almost all light aircraft