Only prob I see with the 2 1/2 is that more of the probe surface will "see" the coals in the gap between the water pan and wall. Could cause higher readings than actual grill temp.
When you go to drill, use some blue painter's tape over the drill spot to avoid chipping the enamel.
Weber Spirit S-320
Charbroil Oilless Turkey Fryer
Wife LuzziAnn 1956 model.
Superfast LSU Purple Thermapen