Leaks are generally bad news if they are between the engine and the
catalyst. But it depends on where the exaust leak is.

If the leak is on the catalyst side of the oxygen sensor and some distance
from the sensor then it probably does not matter.

If its on the engine side of the oxygen sensor then you got problems. The
reason being that the sensor responds to oxygen in the exhaust. If you have
a leak, it measures 'high oxygen concentration'. The engine control module
sees this and says 'ah ha ...too much oxygen, I need to add more fuel' -
result is the engine runs too rich. You can confirm this by looking at the
colour of the spark plugs and they'll be dark brown/black.

I think it'd have to be pretty rich to show the behaviour you describe
though - those symptoms can also be caused by insufficient fuel.  It'd be
worth checking the colour of your spark plugs. They should be a pinkish
colour. If they are white it's running too lean i.e. insufficient fuel.

  steve