I've been running a 28NC HQI for almost two years now. Heat and noise are my biggest complaints. I bought my setup from Nanotuners so it also includes 2 18W PC's...it's a great lighting option, but heat is a problem. The built in fans are noisy, and of a size that I haven't found a good upgrade option (like Silenx). I run a 1/15th HP chiller that is barely adequate for the job. My halide comes on at noon, and by mid afternoon the chiller starts running almost constantly until a couple hours past lights out (10 pm). I've place a fan in front of my chiller to help, but my temps still swings from 76 degrees overnight to the low 80's by late afternoon. My house is generally no more than 68 degrees...even so, running without a chiller is not an option. One issue to consider is how big a pump for the chiller you can get in the back of tank...because I'm still using the media rack, I could only squeeze in an MJ1200...should be adequate flow for the 1/15th, but I don't think that would cut it for a larger chiller.
Running a fan across the back of the tank didn't work at all for me.
As for other mods:
replaced return pumps with MJ1200's
Added Hydor flo deflectors to return outlets
Removed sponge, carbon ceramic rings (running blue pad, floss and Chemi-Pure Elite)
Added CPR medium HOB refugium (plumbing mods required to make this work)...I run a tunze 9002 in the CPR along with a small PC light and chaeto.
I also run a Koralia Nano and an MP-10 in the DT.