In the past, labels (above items and people) took up a lot of memory regardless of whether you had them set on or off; that is no longer the case. Most of the other memory problems are problems in Crystal Space instead, and I don't think switching to Ogre will do a whole lot. Other than that, it really depends on what version you played in the past; 0.4.01 is much faster compared to previous versions.
If you really want to do something with PlaneShift-the-engine, the first thing to do is join #planeshift-build on Freenode and compile it. Unlike WorldForge, PlaneShift-the-engine actually has something that works, as well as a convincing flagship product. The reason it discourages forking of content is that it wants to succeed where many others have failed, but there's nothing preventing anyone from making their own product using PlaneShift-the-engine (other than it's just a really huge task). Another MMO framework you might have heard of is Arianne; it's been around since 1999 and six years later they come out with a Pac-Man clone.