Axle converters just allow 3/8" axles to fit into 14mm dropouts. They don't change the strength of the axle. 3/8" axles just aren't as strong as 14mm axles, so that is the problem when you are doing a lot of grinding.
Heck, even 14mm axles get messed up when you do a lot of grinding.
If you aren't up to getting a new 14mm hub, then the best option is to go to www.danscomp.com
or one of the other mail order companies and order a quality 3/8" axle from Standard Industries or Kink. Those axles are (literally) 3-5 times as tough as 'normal' 3/8" axles that come stock on your bike.