OK...there is nothing wrong with using a Z6 or just about any modern sport bike tire on the FRONT of an LT. According to the Metzeler web site, the Roadtec Z6 has a weight rating of 58, just as required in the LT manual. The REAR tire on the LT is where we need an 'unusual' weight rating that many (most) manufacturers don't produce.
Frenchy, I think your problem is the mileage on the front tire and the air pressures you run. 2.5 bar is 36.2 PSI, 2.9 bar is 42.5 PSI, with 1 bar ~14.5 PSI. Here in the US, most LT riders get better mileage and useful life from their tires by running considerably higher pressures than you posted. The general consensus is 42 PSI front and 46-48 rear, or 2.5 bar front and 3.2-3.3 rear. These pressures seem to delay the onset of cupping and other wear patterns detrimental to ride and handling.
The Z6 is also a more sport oriented tire with softer tread compounds than you will find on a 'touring' tire. 6k miles on a tire such as this on the front of an LT is pretty decent mileage. Replace it with another one, or something similar, raise your air pressure a bit, and I'll bet you will enjoy another 6-8k miles.
I have run Michelin Pilot Road, Bridgestone 020, Metzeler 880, and Avon all successfully on the front of my LT. Currently I am trying a Conti RoadAttack and it is the best of the lot so far. My only hesitation recommending it is that it grips so well that I find it hard to believe that I will get very long life from it. Time will tell.
Good Luck!/Bon Chance!
Get busy livin' or get busy dyin'!
2003 BMW K1200LTC "El Gato Grande"
1993 Suzuki GSXR1100W
1978 Suzuki GS1000C 'Project'