Uggla is a 30-HR, 100+ RBI bat who i would assume would perform even better in a stadium that benefits him more and with a better lineup.
However, Uggla is not cheap, and as noted above, it'd be a butcher job to absorb his and Lowell's contract for an upgrade that wouldn't be as significant as say, Adrian Gonzales or Prince Fielder.
Also, Dan Ugglas hasn't played 1B consistently before, a much less tolerable position for defensive shortcomings than LF. Specially in Fenway.