Freewheels, by their nature (springs forcing pawls in contact with inner hub) are going to wear out, so I wouldn't worry too much about a bit of roughness and noise.
It's more a roughness with the way the chain pulls through the sprocket and chainring than a roughness with the freewheel itself. The increased tension just seems to mean the chain doesn't mesh quite so perfectly with the teeth. As I say, it's not really noticeable while riding, but you can feel it when turning the cranks with the bike up on the stand.
Or a White Brothers one if you have more money than you know what to do with!
I'm getting a White Industries one after payday. Silly price but everyone seems to rate them and the outer sprocket can be replaced without sacrificing the inner freewheel.
I can only think the current chain is slightly too long, given the fact that the wheel is already pulled nearly as far back as it can go in the dropout and I've still got over an inch of play at the mid point.
Currently everything is running smoothly and I have no problems with chain coming off etc. just a clunking noise coming from the freewheel which is there regardless of tension, which others have reported with this particular component, and which is (I think and hope) due to poor bearings.
So my plan is just leave it for now and see how things sit with more tension once I have installed a new (non-clunking) freewheel and new, slightly shorter chain.
Cheers.