It is and always will be a performance CAT A aircraft.
Why? It clearly says ".... must increase category when actual handling speeds are in excess of those for category (based on Vat)"
So if the C172 is Cat A based on its Vat, but you intend flying to speeds that would fit it into Cat B, then you can go ahead with the full support of the AIP/Jepp. The only catch is you can't pick and choose in flight - if you want to fly a Cat B final approach speed, you have to also be able to maintain all the other Cat B speeds.
"An aircraft must fit into and be operated in accordance with the requirements of only one category"
I doubt you can argue to be Cat B just because you felt like flying the final approach faster.
The text of the AIP/Jepp doesn't support that assertion. You can, and many operators do. The catch is you have to be able to maintain all of the Cat B speeds - not just the final app.
I have had the misfortune of many hours IFR in G1000 C172s and always found it sat very comfortably at the upper end of Cat A. Cat B would require holding at 65+% power to maintain the minimum IAS.
To the original poster, please get yourself some proper instruction on the C172 and particularly the G1000 before blasting off in it IFR. You
will get yourself into trouble if you are unfamiliar with the glass and you
won't see it coming. Glass is a very efficient weapon in the hands of a poorly trained user.