I know you get a increase in velocity from polygonal rifling, And since i want one of thier barrels for thier 308, I felt like i would get a full powder burn going with a 16' barrels.Would I be wrong? should I suck it up and go with the 18" barrel anyways?

Also, would dimpling the barrel reduce the weight that much?

I think 18" would be better for a .308 but 16" will work too. Really depends how far you plan to take it with your concern for velocity??? Poly barrels do work but the increase isn't mind boggling by any means. Complete powder burn is definitely more likely in a 18" and probably a better choice regardless of rifling.

My calculated MV was about 80 fps more when I switched from my LMT MRP CL 16" to my BHW 16" SS .223 Wylde. But I think it was the combination of being a .223 Wylde chamber, SS barrel and polygonal rifling. Not just the polygonal rifling.
And getting your barrel fluted will take off some weight, were talking a few ounces, which may be nice, but again you run the risk of inducing stresses in the barrel which may deform the rifling. Im not an engineer but this is what I was told (this is what I took away from him) from a rep at Bartlien. To me it makes sense have a barrel fluted before a barrel receives its rifling.

My 24 inch .30 HRT on a BHW barrel blank is 150 fps faster than my 16 inch traditional land and groove .30 HRT with the same loads on the same day.  Polygonal significantly faster than land and groove, I don't buy that for a second.
Also, my HK USPCT .45 shoots the same ammo to the same speed as does a Jarvis traditional land and groove barrel of the same length in the same pistol.
A USP .40 with the HK barrel is not any faster than a USP .45 fitted with an EFK .40 coversion barrel of the same length.
Three guns, three calibers, all show not a lick of difference in velocity that can unequivocally be contributed to polygonal rifling.