Stick to 2.5" piping, that will be more than adequate.
In terms of aerodynamics, the best angle to reduce turbulence and hence restriction is 4 degrees (involving lengthy discussions with an expert aerodynamicist). This is often imparctical, so the best suggestion is to make the transition as long as possible, and ensure that the pipes are lines up as best as possible.
One thing that may help, is to reduce the inlet/oulet to the I/C to 2.5" by some new pipes being welded on, as this may help in positioning of pipes, cooling efficiency and ease of plumbing, and don't need another 2 of those pesky reducers.