No, aspect ratios are fixed when in optimized mode. The aspect ratio varies by line mode, because of the multiplication factor used.
The one closest to 4:3 would be x5 mode (it is a bit too wide), x4 is 1:1 (i.e. 8:7), x3 is even wider than x5, and x2 is ludicrous wide.
You can see the horizontal multiplication factors used for each mode at the bottom of the optimal timings wiki page:
It’s a shame the table is kind of hidden away like that actually, because people ask this kind of question all the time.