Thieves World and Warhammer are both high fantasy. Both have world-shaking magical power in them. Low fantasy would be like A Princess of Mars, Gor, or that recent King Arthur movie thingie.
Actually, I think most would put them in the low-fantasy camp. In fact, there was a Dragon from years back that had an article called the Highs and Lows of Fantasy that showcased the two styles. TW was in the low fantasy camp.
Low Fantasy doesn't mean magic isn't powerful. It is much rarer and certainly more dangerous than magic is in High Fantasy. If you read the sorcery & campaign essays in the Conan RPG, they highlight this fact rather nicely.
I would submit, however, that Warhammer Fantasy Battles tends to run in the High Fantasy camp. Since WHFRP shares the same setting, it could be run in either fashion. However, the gritty survival theme is much more appropriate to low fantasy than high.
My 2 cents anyway.