I am having exactly the same problem now in my 2006 GT 150ps which I chipped to 180ps/400Nm few days ago. It has 125k miles on the clock and I got it 3 months ago.
Before the remap, the clutch didn't slip at all although it was 'catching quite high' and was fairly heavy to operate (although I have never driven other JTD so can't compare).
When I was coming back home, immediately after the remap (different city), on the motorway, on 4th, 5th and 6th gears....if I bring the car to speed so that the rpm is around 2000 and I floor it....then the car will initially 'go'....then slip around 2200-2400, rpms will increase rapidly by about 300-400 without car accelerating and then it will catch back again and car continues to accelerate.
Very common complaint that the clutch starts slipping shortly after a remap/chip-tune, especially when it's an older clutch already.
Peak torque on these engines is about 2700rpm, but it build quickly from 2000rpm as the turbo comes on boost.. Not surprised you're breaking friction when you give it a poke at 2,000-2,400 rpm...
So your mechanics are essentially correct - you need a new clutch if you want to keep the chip-tune and drive the way you're driving.
you might be bale to nurse the car around by being gentle with it and deliberately backing off around 2000-3000 rpm, but the writing is on the wall... It'll need doing eventually.
DMF can be inspected once the gearbox and clutch are off and out of the way. Expensive repair job if it needs replacing though.
You could looking into the Autolusso SMF/GTA Clutch conversion if you're keen - it's a well proven flywheel/clutch combo on these cars that can take really high torque loads (ask Damien about his journey of discovery with clutches!).. I think initial outlay is similar to an OEM clutch/DMF, but the flywheel can be re-skimmed/surfaced if it needs it so should be a cheaper route permanently...
Cheapest option would be to remove the chip-tune box, use the car as it currently is and save up for a new clutch in the future!