AFAIK 5:4 is just the lowest ratio they've tested. Personally I use table sugar (1:1) and can sustain rates above 100g/h. Haven't hit the ceiling yet, don't really feel the need to explore where that is yet because exceeding the absorption rate comes with the risk of diarrhoea which is bad at any time but especially when you're in the middle of a training session and who knows where the nearest toilet is.

Gut training is consuming large amounts of carbohydrate (preferably in the same form you intend to use when racing), yes.