The mod is called, turn the screw in the base clockwise to get more resistance? Seriously though, TM said in their documents with the TH8 RS that tightening it too much can cause problems. Also I haven't read about anyone doing any modding work on it.
So feel free to open it up and see if there is any modding possible. Start with removing the shifter plate and then the screws of the plate below that I suppose.
Which cars do you drive mate? I have a Lancia Delta HPE from 1999 and it's a 5 speed manual and it's not stiff at all. It behaves differently then the TH8 RS though, but they are equally stiff. My cars stick doesn't pop into a gear like the TH8 does, at least far less noticible.
Maybe your transmission oil is getting old or the wrong kind is used in your car?
Oh yeah, a vans and trucks often shift a bit more heavily then a normal car. Drove a Nissan Vanette once, so I kinda now how it feels. The old Mazda... dunno... sounds like it needed an oil change to say the least. lol
At least I see I´m not the only thinking the TH8 is bit stiff.
Today I received my second unit. It is a bit stronger than first one (or is it placebo, and that's what I think... ), but still far of a real car shifter feeling.
TH8 RS shifter is made for durability and fast change, clearly.