I ran a TJT for a while with modified weights and the stock rear spring. My bike never gave me the RPMs to variate the stock weights. The only problem I really encountered was it really wore out my rear pulley. The spring still worked- but as the days went on it got slacker and slacker... Till I got a euro pulley. You can also just crack open the rear vario and stretch the spring back out, too.
I would listen to Peter on the issue of switching the springs. Don't do it.