Terry,
At this moment, StealthPlug isn't a recommended ASIO device for RiffWorks. The lack of a recent driver might be one of the reasons behind this.
There are a few people on the forum interested in trying to use one so if it works for you, let us know what you find out.
I'm surprised to hear you say that latency is a problem with GuitarPort. Line6 have done a damn good job at getting it right down to something pretty well undetectable (at least to my ears

) Are you using your GP with the latest GearBox release?