i have 3.45 watts/gal right now on my 55 gal tank. if i were to up the wattage to, say, 4 or 5 watts per gallon what would the benefits of this be (provided i match the light with enough plant nutrients)?
Agree with error.
At almost 3.5 watts/gal., especially if it is CF, you can grow just about anything.
Higher wattage will increase variety of plants and give redder reds and faster growth.
With that, however, comes a host of other issues including tank balance, a lot more work, and a lot more pruning.
IMO, it's not worth all the hassles involved, and should not be considered unless you have a thorough knowledge of tank management.