You should never sheet in hard on the spin (unless you are reaching). It should always be sheeted so it doesn't fall in, even in heavy air, you will notice a huge difference in sheeting out that last inch.
Maybe your main is not sheeted correctly to your spin. Personally I use tell tales a the leech of the sail top 3 battens. I let out the main untill those are flying (this will seem quite far out, but works ok for me (mast will look quite bendy). This will generate more power and lift in the main and might work better for the spi too.
Also talk as skipper to your crew when heading up or bearing away, and crew should inform the skipper how the spin feels.
Finally go your own course. If your spin isn't made to sail high and fast don't do it, because it will not work.

Just sold a new Glaser spin to one of the Blades here, I'll let you know my experience.

Gill