Definitely not normal, and definitely not 511C. Are you using the Toshiba-supplied driver or the latest AMD mobility driver?
NOTE! Certain laptop configurations are not compatible with AMD's Mobility Catalyst™ Driver.
AMD provides generic graphics drivers for most laptop brands and models with the Mobility Radeon™ HD series of graphics cards.
LAPTOPS FROM SONY, TOSHIBA, and PANASONIC will not accept generic drivers. Get drivers from manufacturers support site.
Faulty thermal sensor, ignore it.
I still recommend checking the teperature, cleaning the internals and especially applying new thermal grease.
Like 2/3 of laptops have cooling somewhere between "insufficient" and "designer of that thing should be shot on sight". So most GPUs are overheating constantly (>80 °C is NOT good temperature and >100 °C is very bad), it gets even worse when ppl don't clean the fins and replace that grey stone-hard chinese thing (supposed thermal grease) sothey finally die within 3-5 years. RoHS (and similar packaging cr*p) plus insufficient cooling = laptops are dropping like flies.
But using 40W GPU in a laptop is pretty stupid too.