The 17.6 are similar to the 16.6 tips. None were created before improvements in winglet design so they tend to provide improvements at lower speeds but drop off at the higher end. I can’t remember if the 17.6 C had the same 980 pound limitations as the 16.6 on the B models. This limits the usefulness on stronger days.

The tips are better if they have winglets, but most of these were modified after manufacturer. I would prefer to fly a C in 15m with Maughmer winglets than 17.6 without winglets. The winglets help the Ventus groove and improved the stall characteristics.

Unless the glider has the tips when you buy it is very difficult to find a set later.

The longer tips are nice in lift less than 3 knots, for longer glides at slow speed, and floating around on weak days. Without winglets the glider does have a shorter fuselage and rudder optimized for 15m so yaw can be a factor.