I wanted to try T-5 mostly. Also the single MH is not ideal for a 36" tank. The Fishneedit light was so cheap that I was willing to take a gamble.
I've always been a bit tempted to try T5 myself due to the volatile nature of MH, but I'm too attached to the shimmer. Nowadays if I had to try anything else it'd be LED, still has the shimmer of MH but lacks the heat. Also, I've consistently been skeptical about those fishneedit fixtures, I would not be surprised if you were right about the bulbs in it. I know they're not bad fixtures because I've never really heard complaints about them, and Barr loves his fishneedit MH, but I feel if they wanted to cut corners to get those low prices, it'd probably be on the bulbs. Pure speculation
And at 12" off the surface, especially with the flourescents, that fixture should be OK for 36" IMO. Grand solar I is pretty much the same exact thing, and what amano uses on all his 90P's, 90H's are another story though. It's really when you get to salt water that halides need to be used for 24" and not much more. If you can't turn it around with the T5 I'd consider giving the old fixture a chance, if the T5 isn't doing it you don't have anything to lose in trying.
My tank is 30" and I have no problem getting the corners high amounts of light even with the 150w MH at 8" above the surface, and thats without the fluorescents. Right now it's 11" off the surface until things have settled in.