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Like many, I use a piggybacked 80x400 guidescope and guide cam (QHY5L-II) quite happily at 1625 focal length with no major differential flexure problems.
I think my guide cam sensor is the same as you're contemplating, so I'm pretty confident that with a f/5 guide scope you should have no problem finding guide stars at 1.0s exposures or less.
Autoguiding with a guide scope will add a couple of kilos though so your mount might struggle to track reliably enough for imaging if 10kg is its max load.
NEQ6/HEQ5 owners usually go up to about 2/3 of maximum payload for imaging. For example the NEQ6 specifies a 25kg load for visual, but 18kg for imaging. HEQ5 is 15kg visual, 11kg for imaging.
I don't think anything terrible happens when imaging close to the maximum visual payload, it's just that accurate guiding gets more and more problematic.
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