I can tell you I'm 99.9% sure you don't have to worry about the rail flexing, even at 1500mm... the 16-60mm is TOUGH stuff! Be sure to sandbag good though and use a solid tripod.
Oh... and get the feet... you'll love them!:happy:
Gonna be using it with the manfrotto 028b legs, do you think that 1800-2000 is a stretch for flex or is that just a stupid length for a slider? Ive never had a slider but 1000 seems like it would be kinda short. aside from the tipping over and the possible flex to I open myself up to other big problems (besides fitting it in my location) for going larger?
If I planned on going a little longer and possibly putting a 300 or 500 on it later is the 150mm (or larger) carriage a better choice or even the fatter rail? I dont wann have to drill the rail if I dont have to and I rad about the twin holes on the bigger rails.