Hi Snatcher,
I don't think it matters that much if they are a bit "saggy" at the back, its how they perform in the water that is more important. They look like they will have a pulsating action which would be great.
I have tried tying my own pike flies out of rabbit fur, which look ok when dry, but are very difficult to cast when wet. With your flies made out of synthetic fibre, the should be much lighter.
Any chance of a step by step guide on how to tie your flies? Im sure there will be a few people on this forum who would be interested.