Looking to get into the 18V batteries - advice?

[mino]

With the TSC-55 it’s already a hefty tool with two 5.0 batteries, the slight deduction of weight isn’t a great concern. With the 5.0 batteries it’s closer to the weight of the TS75? The hand tools like vecturo, drills and tid18 would benefit from the weight loss program. If I’m using the drill all day overhead, on a ladder, outstretched arms the lighter the better I really wanted to run the tsc or the hkc55 on the 4.0 batteries but I took the advice of Festool application support. What do I know about battery technology?

This is always a problem with asking official support - they can safely recommend only the optimal solution else they run the risk of people being unhappy with the performance.
Even doing a full-on explanation can even be seen as condescending, if not confusing, by the customer. Been there, done that ..

The 21700 cells used in the new 4.0 bats have a much high rated current per cell than the 18650 cells in the "standard" batteries have.

Would I get these for it if I planned to rip ply all day on a site? No. I would get the 6.2 ones.
Would I prefer them for occasional hobby use or limited use during fitting work? Definitely.