Randomized controlled studies with meaningful longitudinal results aren't feasible. You'd have to randomly place kids into specific controlled environments for long periods of time and prevent them from exiting those environment when there are problems. You also can't control for family mobility and too many other factors.
I'm very skeptical on most research relating to decisions for individual gifted students, because there are such wide variances between different levels and such distinct difference amongst the gifted population and a large overlap with LDs. It would be hard enough to find adequate population sizes without also making sure you have enough depth in say sub-categories like: balanced IQ scores, nonverbal significantly larger than verbal, high IQ with average PSI, EG with anxiety, HG with visual processing challenges,... etc. etc.
All that said, I'd say this site (Davidson) has about the best collection and well organized of studies that are out there. If you haven't explored it, it's
http://www.davidsongifted.org/db/
