The risk is that 'IF' the holding tank liquid level is higher than the lip of the
toilet bowl 'THEN' you could conceivably induce a reverse siphon of the liquids in the holding tank back into the bowl. (Most
marine toilets use a 'duck' valve to prevent backwards movement of the liquids (well, and solids...)) if this were to happen you would be wiping and wiping and wiping - and that's just the disgusting part...
My boat is a
trimaran so I don't get the severe heeling ('severe' is admittedly a relative term here, one man's ceiling, another man's floor...) you might/do.
But on Ceil, the head (toilet bowl lip-line) is unfortunately AT the normal waterline - so when I was at your same point, (tired of pumping the flow UP hill into a tank) I removed the anti-siphon loop...
Luckily I was on-board two days later when a storm came through - the duck valve in the head probably slowed thins down but did not stop the flow... I have a
shower well built-in to the head so the waste was 'only' 3 inches deep but it was still pretty '^^&itty' if you get my drift. I woke to the smell but also heard the sloshing. If I had not been on board the boat would have been host to the entire contents of the holding tank because once a siphon starts it doesn't stop until the liquids are all down-hill...
Lessons learned:
1. check the anti-siphon loop valve (if it's not clear it will not stop a siphon starting)
2. don't remove the loop for any reason.
3. '$$%%@ it' stinks, but it stinks WAY more when mixed with
salt water....