Pinoy Cali, I woud defenitly go with 24.
I run two 24 full moded engines in my rc8t(I have two cars, one for racing,one for practice)and realy like the power of 24, if you dont need so much power you can still geared up or install smaller venturi on it and you get extra runtime vs the 21 with the same power with standard venturi, so why use 21, if you can get the same or more runtime with 24. And yes, i also run sirio 2056 pipe. I get 10-12 min. of runtime, depends on track. At first I wanted to buy extech 2056 pipe(but its all the time on back order at amains) so I took sirio2056 pipe, and Iam realy happy with it, and I realy think its the preety the same pipe as trinity pipe.
hope this helps