15/18m Construction and Performance?

An owner of a 15m with 18m tips told me his glider was designed as 18m and the manufacture cut the tips to make removable and that it did not fly well as a 15m.

My question is this common or glider specific? How do manufactures design a 15/18m? Do they start with design of 15m and add 3m and then test performance or design 18m and cut down? Or do different manufactures have different technics? Can a 15/18m have equally performance or does one configuration suffer and that's the price one pays?