Since I plan on trying to get to 300hp, what does that mean for my ATTS, transmission and ECU?

Auto or Manual tranny?? You'll need some sort of EMS upgrade whether it's a piggyback system or a stand alone unit. Tuning is your best friend.

Manual. My understanding was that once you got around 180lb's of torque that the ATTS would break down. I also understand that the transmission in the SH is different from the base model. Does the SH tranny work without the ATTS? Will the ECU work without the ATTS?

Your ATTS unit will start to intermittently fail after a certain ammount of torque (i think it's in the 180ft-lb area). Removing the ATTS involves fabbing/buying a plate that mounts to the engine block where the ATTS belongs, that will allow for the base prelude intermediate shaft to be placed there. The ATTS unit sits in the area that the intermediate shaft sits on a base lude... unfortunately, the bolt holes for the ATTS unit are different from the bolt holes for the intermediate shaft, so an adapter plate needs to be fabbed that will allow you to mate the intermediate shaft to the engine block.

Tuning will have to be done by either standalone ems or a chipped OBD-1 ECU and tuning software (like hondata, chrome, neptune, etc...). Piggybacks (like the V-AFC) are not wise to use if you are aiming for 300 hp.

Transmission will be fine as it is... except if you add crazy torque you are gonna want a higher stage clutch with a higher clamping power.

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Would the car still work even if the ATTS is blown? Sure it's 60lbs of dead weight but it's a lot cheaper.

I believe yes. If I remember correctly Pooponurhead has been running with a blown ATTS computer for a while now.

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Yeah it'll run just fine. It'll actually run better if it is completely failed than it would if it was experiencing intermittent failure... kind of like ABS. You'd rather have it never work than just crap out on you from time to time unexpectedly. (which is what happens when you take it over that torque limit...)