recommendations with downpipe

Hello friends, I have the opportunity to get a new dowpipe and wanted to ask opinions, my idea is to make approximately 2.5 "(60 mm exactly) like me anger this measure? I do not want to lose power at low rpm, so my idea is to follow the line from the dowpipe in 55mm, a recommendation to bring benefits to my car? a greeting!
i use a 2.5" down pipe on one of mine.
2.5" from manifold to the 200cpi cat then dropped to 2.25" for the rest of the system on the vectra B with a light tune (cams and minor head work)
upgraded to 2.5" from manifold back now its in the corsa C
using 3" to cat on the supercharged one.