I just drystarted a 3g rimless cube and purchased a single 3x3 (9w) GU10. the 30 degree optics were a bit to narrow so I ordered some 60 degree optics. It lights the cube pretty well but I havent had it on there long enough to determine the light levels. Also the 3x3 is HOT so im guessing im not going to get the 50,000 hours out of it. Big deal, $4.50 a pop id bee happy if it lasts a year.
Im an active member in nanoreefforums. a guy on there named tinygiant started a HUGE thread on them. plants don't need nearly the power SPS corals do so i really don't know. Im going to put 7 or 9 over my 12x12x12" cube (7.5g reef)
I think his formula for a reef is length of tank divided by 2 minus 1. So if it were a reef youd need 14 to 16. (30" divide 2 – 1) as a GUESS id say 8 or 10. but please do more research I am only guessing and telling you what I would do since there is not enough data out there on them for planted tanks. At least from my research.