Ativan is pretty interesting in that it doesn't use any of the typical drug metabolism pathways, and although I don't know whether or not this will cause a huge variance in patient response, it implies that, just because Ativan doesn't seem to be working ideally, doesn't mean other benzos won't. It takes me about 4mg of ativan to get somewhat desirable effects, but even 2mg is a pretty high dose for most people.
The only real problem with taking bigger and bigger doses is that dependence becomes more and more serious and dangerous, and constantly asking to increase the dose is probably making you more likely to be susceptible to this, in your doctor's eyes, as you are pretty much mimicking drug-seeking behavior (even though that doesn't seem to be what you're actually doing.)
I've found that, with a lot of psychiatrists, it's a bit of a turn off for them to have patients repeatedly asking to increase the benzos. It sucks for the people who are legitimately unresponsive to the drugs, but the doctors are trained to be vigilant about addicts and it's inevitable that some innocent people are going to get burned in the process. They'd also rather not be responsible for playing a part in creating a new addict.
I'm not sure exactly what signs you're looking for to know if it's working or not, but benzos are so subtle that they often work without realizing. Weight DOES matter, and although I can totally relate to the feeling that normal doses of benzos just don't work, I wouldn't try out 2mg right before a presentation. You might not even realize it while on the stuff, but higher doses of benzos could make you look and talk like a drunk... not something you really want your teachers/co-workers or whatever seeing. Ativan is also pretty bad among the benzos in terms of memory impairment, and you will probably not be at your sharpest. Personally I wouldn't double up on a dose during a presentation until you're really aware of how such a dose affects you. 1mg may not seem to help overtly, but should definitely still be able to keep you from completely losing it while presenting.
As for your pdoc... if he said that Ativan doesn't work for everybody, he shouldn't have a problem letting you try something similar, like Klonopin (clonazepam). I have the choice between the two, and I stick with klonopin because I find it more effective, and it's also longer-acting and doesn't impair memory to the same extent. Maybe you'll have more success with it.