The latest heated bed does seem to heat more evenly than the taz 3 bed did, though mine also may have been damaged at that point because it didn’t heat well in one loop at all. comparing with the infrared camera though there was definitly a difference for the better.
A taz 3 converts to a taz 5 well. You can get most of the parts to do so from http://i-t-w.com/parts . The most difficult part to source is the X end plates, which seem to be in stock at the moment so I would grab those immidiatly if you plan on doing the conversion at any point. The leadscrews and leadscrew nuts are the next hardest to find.
It is definitly worth either upgrading to 5 spec, or sourcing 12mm rods and bearings and going to Taz 6 prototype spec, but that design isn’t finilized yet. There are some advantages to going with the 5 spec, such as the ability to use the openbuilds modifications. To go wtih 6 spec you would need to also replace the front Y axis endplate and the bed plate. You can get most of the way to 6 spec if you omit the mini style leveling without replacing the bed mount plate.