Hills, atmospheric extinction and light pollution all make it harder to get to the theoretical limit. From my 55 deg latitude I've had views of M4 and NGC253, both about 10 degrees above my horizon, but neither was particularly inspiring - and they're showpiece objects. I've seen them with smaller scopes from more southerly sites and there was no comparison.

Can anyone offer a rule of thumb for the magnitude loss close to the horizon? (i.e. something more rough-and-ready than the extinction formula, which isn't much fun to work out in your head on a dark night).