Does the shared server have every thing you need? Is it financially feasible to upgrade? How are your load times on the shared server? Do you expect a spike in traffic? Do you have costs for bandwidth?
80k visitors is OK but not huge load on most servers, now if the hosting company says its too much then you might want to see about getting bigger shoes.
To some extent you must consider the bandwidth use; is the forum full of media files etc. or is it mostly text like WebmasterWorld?
The traffic count does not sound excessive; however the real measure is response time. Do some benchmarking against other major forums and figure out whether you are slower than, similar to or faster than them.