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In theory, nebula filters like the UHCS are more efficient at F10 as the cone of light is more parallel, and as the filter is an interference based filter, the passband will be more accurate, at very fast f ratios the light hits them at more of an angle, meaning they arent as efficient. Grab the UHCS filter and tilt it while looking through it in daytime, you will see the colour change.
Anyway your tracking is certainly handling F10 well. If you go 4 times longer (or 4 times as many subimages) you will still get same brightness as shooting normally at F5. E.g. shooting F5 for total of 15 mins, can shoot for 1 hour at F10 and get same intensity (but better images due to better filter efficiency)
Scott
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