Some people feel it reduces engine temp to remove it. would like to hear opinions on this issue. Does it help or hurt? I feel porsche engineers now more about these thing than I do. Recently bought a 75 and PO removed it and feels it runs cooler in stop and go traffic here in California's great valley where temps are frequently over 100 degrees in the summer. The part came with the car and trying to decide if I should reinstall it.

Do you guys even have rain anymore?

I think it's been proven temps don't decrease if you remove the rain tray anyway. Not on a stock-ish 914-4.

I would opt to leave it in if possible. That much less water getting on electrical do-dads etc.

I recall hearing the same about temps from tests with it installed vs not installed.

The effect of not having it installed is that things in the engine bay get more wet.

It may be slightly cooler. However, if the car is outside you can get more dirt and grime not to mention water in the engine compartment. You have to weigh the benefit / reward.
Personally, I'd leave it in, you can get more cooling putting an LE airdam on. Not to mention the air diverters. If your temps are critical, you probably have a blockage at the oil cooler, and you will need to clean it out.

I could see the tray maybe increasing intake air temps a little by trapping heat in the engine bay, but it's hard to imagine it is restricting fresh air flow to the cooling fan enough to affect engine temps.