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Nigel,
I think you're pushing the limits...
Rule of thumb: optimum imaging focal ratio = x5 the pixel size, in micron.
For the ASI 174 (5.86micron) this gives around f29, close to a x3 Barlow on the C925.
Running the mirror through the full focus travel (30 turns or so) re-spreads the grease between the mirror boss and the baffle and usually reduces the slop.
Even if you do fit an external focuser you still have the potential of mirror movement unless you use the fitted mirror locks.
I use a C11 (no mirror locks) and regularly work at f10 for spectroscopy. I can easily focus - using a JMI focus motor on a 20 micron slit with little focusing slop.
I have 5 x 5Kg counterweights on my NEQ6pro - no issues, no drama.
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