6500 rpm's itself won't hurt the engine. A bad tune that runs lean and causes detonation is more dangerous at 6500 rpm's. Make sure you don't hear any pinging. The pinging wouldn't be caused by raising the limiter, it would be from a lean tune.
Eventually....may I ask why? Stock HP peaks around 5,900rpm. Anyway If you get mods that make HP that high up include a set of underdrive pulleys, the lower weight and mass allows for high revving.
Brenspeed shipped my SCT with the rev limiter set at 6800 and when I asked them if it was safe they said it was. I trust them enough that I left it there.
Yes it is safe, if the tune is safe. Like said above, if you aren't pinging or going lean, then 6500 RPM is fine. I used to shift my 01 at 7000 RPM on the stock block with my blower making 14 PSI right when I shifted. I did that for over 2 years and never had an issue.
My SCT from Brenspeed also came set for 6800. Mine never did ping the 7 or so times at WOT down the track or the several times on the street The second time at the track I actually dropped it to 6500 and slightly improved my times. Spyder 7724 was telling me it works good with the auto to short shift second and then run the rest way out, but without a custom tune loaded, I can only select shift RPM for all gears. The only thing I can change gear to gear is the shift pressure.