MS SQL 2008 Express - Database Tuning Advisor
As long as you don't enable any features that aren't supported on Express Edition, you should be fine. For example I think you can use DTA to recommend table partitioning which obviously won't work on Express edition. The other thing to think about is that the server running Enterprise Edition might have a lot more memory or CPUs, so the optimizer on that server might make much different plan choices than it will on your web server.