Yes,
far better. The GT 755M is the same silicon and architecture as the GT 750M (hence why they only differ by one digit). The only difference is that the GT 755M is clocked slightly higher.
Look at the benchmarks towards the bottom of these pages:
http/www.notebookcheck.net/NVIDIA-GeForce-GT-750M.90245.0.html - GT 750M
http/www.notebookcheck.net/NVIDIA-GeForce-GT-755M.102020.0.html - GT 755M
The difference in frame rate between the GT 750M and the GT 755M in games is only about 3 - 5 fps.
Now look at the benchmarks for the GT 750M SLI:
http/www.notebookcheck.net/NVIDIA-GeForce-GT-750M-SLI.91280.0.html
The difference in frame rate between the GT 750M SLI and the GT 755M in any game is anywhere from 5 to 30 fps in the GT 750M SLI's favor. The GT 750M SLI is more powerful because it's two of the GT 750M's together working in tandem to render the image, hence there's theoretically double the performance, though in practice this performance advantage is less than double due to other issues such as coding inefficiency, CPU bottlenecking, and other things.