Velios did make a 915 side shifter kit. The one advertised on the first AA web catalog page under "transmission" (this page:
http://autoatlanta.com/model/914-0704/Sec3/914-page042.html) is the Velios kit. The major difference between it and the factory 916 shifter is that Velios copied the 901 side shifter design to as large an extent as possible. The cast part that bolts in the side of the transmission carries an oil seal that puts the final joint between the shift rod and the shaft that works the internal shift rods outside the transmission, i.e. not in the trans oil bath. The 916 design had the corresponding seal on the shaft extending toward the firewall, so this last shift joint was inside the trans.
On at least some of the 916 parts, there was no return spring to load the shift lever into the 3rd-4th gate, and both Velios and JaideCar fixed this on their side shifter kits. If your 916 box lacks this return spring, this might be why it doesn't shift as well as you'd like. I have one of the JaideCar kits on a '78 SC 915 box in my car, and it shifts extremely well, I'd say as well as a stock 911SC. I had a Velios kit for a while, but traded it to Scott Yeaman for part of a paint job on my car. For all I know he may still have it, or know where it is.