The Weber Performer is a nice step up and gives you a useful cart with some table space and storage for your charcoal and accessories. While the Performer's included temperature meter is a good starting point, it is woefully inadequate for the home-griller wanting to turn their kettle into a real smoking machine.
Below I will illustrate a couple of installation options for the new temp meter on your Weber Performer (or other Kettle that has an OEM thermometer).
You will notice when you remove the old meter and bezel, there is an "extra" hole that holds the bezel from spinning. If you choose to mount the new thermometer you will want to seal this extra hole with a dab of high-temp silicone sealant to prevent smoke from escaping. The large size of the new thermometer will easily hide the old hole.
Another option (that I did with my own kettle) is to place the new thermometer right on top of the old Weber bezel. It sticks up a bit this way, but I personally like the look.