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Hi Alistair,
This seems about right.
With most scopes (aperture around 300mm, or 12") the main seeing contribution is tilt/tip. Roughly half of the seeing is due to the near ground layer, the other half comes from the mid to high atmosphere.
Using 300ms will freeze some of this, especially the near ground contribution.
If you would go even faster, let's say around 50Hz the improvement in FWHM, therefore guide star wander, would be around a factor 2 to 3 in the visible, or NIR, versus long exposure time (few to 10 seconds). But this improvement will only applied to the isoplanatic patch now.
Using NIR, above 700nm or so is a good strategy which does not require tracking too fast.
With your filter, you should improve (as mentioned before) your guide star seeing (wander) by up to about 60% to 70%
Which means a 0.3 rms becomes about 0.1 rms, or so, consistent with your observation I would say (considering the model incertitude and local conditions).
The turbulence theory provides a nice model, but it is just a model with its assumptions, approximations, and limitations. We should consider its prediction with some degree variability in practice (like the weather forecast).
For instant near ground the turbulence structure could be much more complex and subtle than the model. Be in a valley, or near structures (building, roads...) may impact the results quite significantly.
I understand that your filter uses plastic substrate, how good is the optical quality?
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