Yeah, now that people mention it I do think the VISOR would help quite a bit. Early on in TNG he even says he can detect if humans are lying from the minute physical changes (although the writers clearly drop that later, see his record at poker). Clearly that should be able to pick up all sorts of problems like internal temperature fluctuations and heart rate and in fact it does [The Enemy, TNG]. But what we don't see is Geordi ever fixing those things on a humanoid on screen. So he might be better than O'Brien, but I still think B'Elanna has the edge overall. Even if my logic is iffy (and it is: she fixes the Doctor so she knows about doctoring? Eh) on screen other characters acknowledge her medical skills. Including the Doctor, who gives her genetic re-sequencing idea serious consideration before she reprograms him. Would she be a better medic than even Tom Paris, who has a lot of practice but clearly doesn't like the work? I doubt it, but I'd probably take her in a life or death situation over the other engineers based solely on the primary sources.