I had a Shoestring from GP until the Toledo show and sold it. Don't know why as it was absolutely the best flying plane I have ever had and I've been modelling for over 50 years. I had an OS 60 FSR with a Sliline Pitts muffler and it was perfect. Enough power to be fast around pylons but not overpowered and had a very throaty racing engine sound to it. Only problem I encountered was fitting the upper cockpit to the wing but it wasn't a major issue. I can highly reccommend this airplane and I'm going to get another one.
By the way I'm not exactly a proponent of Great Planes aircraft. I had a Spectrum and it was junk but the Shoestring is very nice.