I would say a sealed compartment might not be the best for it depending on how you have it set up. With some ventilation holes you may be able to get away with it. Just my 2¢.
There's tons of circulation there so I don't see a problem with the ECM location (unlike most closed boxed vehicles).
I wouldn't waste my money on the closed loop system. It runs like crap with that on. The guy that Dyno'd my setup suggested I disconnect it, which I did.
From the link you provided it looks like the 2D system. I don't think this kit has the closed loop system. It would be an add-on, therefore it would run "open" all the time. I spoke with Holley Tech support before I bought mine and they recommended the closed loop system but my experiences...
Straight from the Holley instruction manual for the closed loop kit :
The digital ECU used with the PRO-JECTION 1D and 2D Fuel Injection Systems is designed to utilize a heated, three-wire oxygen sensor to operate the PRO-JECTION system in a closed loop mode. When operating in a closed...
Pro-Jection 2D
I've been using the Holley Pro-jection for close to 10 years now and am quite satified. I have had to replace the ECU once (got the 2D computer as a replacement for the dead 1D).
http://www.holley.com/502-20S.asp
I agree with tuning a few times a year depending on how much you...