Running at 6k all the time might take it's toll in the extreme long run, but I really wouldn't worry much about it. My 500cc Magna V4 ran between 6500-7000(75-85mph) RPM for hours at a time- it redlined at 11,500, and I sent it past 13 more than once because I'm an ass-hat and wanted to see if I could do 55 in first gear, or was too excited that my 500cc was whooping a H-D with 3x the displacement and I forgot to shift.
Granted, I only rode about 9k miles on it but it never gave me a lick of trouble.
Look at it this way...most American stock V8's will spin to 4500-5000 RPM and then blow(older ones), or bounce off a rev limiter(new idiotproof ones). Cruising on the highway my old Ford 351 truck would spin about 2200 RPM at 75mph, or about 50% of it's maximum engine speed. My newer Ford 5.4 truck only spins at about 2100, and hits a rev limiter at 5200. My work has a Chevy 2500HD 6.0, and that spins about 2600 to only do 70, it's got super tall axles. Though I've pegged it's RPM gauge at 6k accelerating in drive...sounded like it was about to explode. Sports cars with high gearing and high-revving motors excepted, most cars run between 40-50% of maximum engine speed to do highway speed.
The Vmax spins to 9500, and a highway cruise at 80mph for me(with venture drive) is about 4500 RPM. Not even 50% of maximum speed, and car engines these days easily see 200k miles. That 351 had 145k on the clock and still ran like new. Not to mention the Vmax motor is so underworked just cruising along. The stupid 4 cylinder SUV's and minivans are WOT going up any sort of highway grade, and usually have to downshift.
