Definetely a big jump in final. You currently have a 13.1 final (87/18*2.72), going to 28t pinion would change it to a 8.45 final.

Going to a 76 spur, would make your final even higher (7.38). In order to not make a real big jump in final with using the 28 pinion, you need to go UP in tooth count on the spur, which may not be able to even happen.

110 spur / 28 pinion * 2.72 trans = 10.49 final
120 spur / 28 pinion * 2.72 trans = 11.65 final
130 spur / 28 pinion * 2.72 trans = 12.62 final

This is just to show you final's that would be close to what you already have. Probably dont even have spur gears that high available? If you want to use the 28 pinion... id get the biggest spur you can get, and maybe go to smaller tires to lessen the load on the motor.