None of my Rubik's brand 3x3s cut corners well either.
I had the same problem back when I could not get a DIY. There were not any mods for Rubik's brands back then.
but now there is this video
http://www.youtube.com/watch?v=YTDblHV-NMU
Hope this helps.
What type of lube do you use?
I put lubix in my transformers 3x3, which turned horribly with CRC and jigaloo, and it made a big difference.
To help stretching, try using a butterknife or something similar to space the pieces.