If I understand you right you are an experienced modeller and you are much more in saoring than in powered modeles. But you can read this on the cover of the manual:
BK / KIT Solius# 21 4264
vorgesehen für den MULTIPLEXBrushless-Antrieb # 33 2660 / 33 3660
Designed for the MULTIPLEXbrushless power set # 33 2660 / 33 3660
Models like the Solius are designed and optimized for the powered version and any modeler who claims expierence in this field should know that running a kit like this as a pure glider will need some modification as the weight of the missing parts (motor, accu, prop, ESC ...) have to be compensated. You cannot really expect any reduction in weight - eventhough the manual says that 200 g are possible.
By the way - and I am not a very experienced glider pilot - most users put in additional weight for appropriate flighing in wind and at slopes.
P.S. If you are English, British or American you let miss all forms of politeness - and I always thought your are so keen on that. The forum is not MULTIPLEX and we deserve a certain politeness even if you are disappointed with a product. Here you are mainly talking to customers (eventhough mostly infected bay the MULTIPLEX-virus) not exclusively to MULTIPLEX-representatives.