how much worse is traction with 18 inch drag radial compared to 16 inch
 

Well here is the question..

How much difference in traction would you say a 265-40-18 drag radial mounted on a 18x9 inch wheel would be over a 255-50-16 mounted on the stock 16x8 FD wheels?

also, would a 275-35-18 be a better option for my 18x9?

thanks in advance!!

forgot to mention the 16 inch drag radial is a BFG comp T/A drag radial and the 18 inch is a BFG G-Force T/A drag radial

Traction should be the least of your worries. Drag radials are extremely sticky when heated, probably even more than slicks! You don't need a wide footprint to hook a drag radial. The main problem you will have is drivetrain shock. Either tire will hook when heated. The more aspect ratio you have on the tire, the better for more flex before the "hooked" tread portion of the tire transfers shock to the rest of your drivetrain. The 18s will cause more wheel hop and shock than the stock ones, and in turn cause more drivetrain "hit", but they will both hit hard on a good launch. It all depends on what drivetrain parts you have beefed up, and what your goals are. Also is this for on-track or off-track use?