I agree with bio. I had both the FX SPK and the Nielsen pin probe to test against my already moded TP and probe. I saw no appreciable increase in FPS over my setup. The Nielsen was substantially beefier than the FX. There is a downside to the pin as well, you loose pellet depth to the leed depending on how deep the skirt is. Pin probes don't always play fairly with all slugs either. Magazine style, slug length, base and shape all play a part in loading them straight and smoothly, or not. FX's standard barrel style probes which mates with the outside circumference of most projectiles, will seat each variety to exactly the same position regardless of base style. Boattails might be the only exception. My pin probes have been sold.
Patrick