AIP ENR 1.5
Aircraft Performance Category
The following categories, based upon Vat (except for CAT H), determine landing minima for aircraft:
CAT: A speeds up to 90KT IAS.
B speeds from 91KT to 120KT IAS.
C speeds from 121KT to 140KT IAS.
D speeds from 141KT to 165KT IAS.
E speeds from 166KT to 210KT IAS.
H (helicopters) see sub-section 1.3.
Note: Vat is the indicated airspeed at the threshold which is equal to the stalling speed Vso multiplied by 1.3 or the stalling speed Vs1g multiplied by 1.23. Both Vso and Vs1g apply to aircraft in the landing configuration at the maximum certificated landing weight. If both Vso and Vs1g are available for an aircraft, the higher resulting Vat must be used.
1.2.2 An aircraft must fit into and be operated in accordance with the requirements of only one category.
An aircraft:
a. may not reduce category because of reduced operating weight, but
b. must increase category when actual handling speeds are in excess of those for category (based on Vat) detailed at sub-section 1.15.
Range of Final Approach Speeds
Cat A 70 − 100 knots
Cat B 85 − 130 knots
Cessna 172 Vat approx 52 knots (stall speed full flap approx. 40 knots x 1.3) = Cat A
I doubt you can argue to be Cat B just because you felt like flying the final approach faster.