I havent much experince with the Par 38 Bulbs,but alot of people seem happy with them,however,they arent the most cost effective to buy from the start,yes they pay for them selves but it takes longer as they have a higher up-front cost compared to a LED fixture that are DIY,or a fixture that has 120 1 watt LED's in it.
I myself would go with a DIY fixture.You can put your bulbs in the order you want them,You can change the optics,which I belive you cant do with a Par 38.
Just look at the diffirence of each fixture and see what you like best.I like the DIY fixtures because I like to work with my aquarium stuff,not have it shipped to me and then its plug and play.
I dont think you would have a problem with keeping a BTA with LED's,but you have to have the right LED's,and like I said,I havent much experince with the Par 38 fixtures,so I could be wrong,I would say call up Bill from Reef LED Light and see what he says.