The 290 head is a closed chamber oval port head. It's an OK head for what it is but most guys will use an open chamber head and enough dome to get the CR where they need it to be.
Porting- if done correctly by a porter who knows his business- can net good results from about any production head.
Most times the easiest gains are made by reworking the pocket just below the seat to remove the lip or ridge that's usually present, and by doing a good valve job based on good valve stem guides. Once the RPM range and valve lift gets up there high enough, more porting is needed to get the flow required to make power at elevated RPM. That's where a guy need to know his stuff- there are no more lips/ridges- it's about increasing the cross section and things like that, that have no visible cues to go by.
So how much will you gain? I really don't know. But I do know the cost of having a pro do the work will quickly exceed the cost of an aluminum head that is already better flowing than anything that you could do to the 290.