It isn't "too high", but must would prefer to stay under the flash point of cotton, which is right around 450F. If you keep your wick wet, you can easily bump it up over 600F without issue. The problem starts if the wick gets dry, above (approx) 450F you can get burnt hits, which pretty much defeats the purpose of using TC. If you are religious about keeping you wick wet, and need to run a Ni build way past 450F, you might as well use kanthal and avoid the extra work that comes with Ni coils.