Total agreement with regards some light twins, Muff.
This discussion started with the Learned Good Doctor asking why he couldn't fly different category speeds in a C172. Changing the approach speeds, however does not change the 'Performance Category'. If flying an IFR approach in CTA, ATC will expect compliance with Perf Cat speeds, but that does not preclude either party in asking for a modified speed profile. (as you said)
The whole 'reason for being' for performance categories is explained in
CAAP 178-1(1) Non-Precision Approaches
WHAT ARE THE STANDARDS REQUIRED FOR THE PUBLICATION OF A RUNWAY APPROACH?
Alignment. To avoid the necessity to conduct turns close to the ground, the alignment of the final approach course needs to be closely aligned to the runway centreline. PANS-OPS design rules permit the final approach course to intersect the extended runway centerline at an angle up to 30° for Category A/B procedures and at up to 15° for Category C/D. The final approach course also needs to intersect the extended runway centerline at a sufficient distance from the threshold to allow a turn onto the runway heading to be completed safely.
Edit.
I also commend Muffs and FTDK's warning to the original post re caution in a G1000 C172 without being on top of your game. I fly 'Glass' for a living, and the glass 172 needs a lot more attention than an A380 does.