this is where the witchcraft comes in. The pipe size before the turbo would have to be determined by the size of the inlet of the turbo. Considering how limited in pipe size your are before the turbo (due to space), I woulnt think that this would be a major concern. Sure, consider it, but its the pipe after the turbo that would have the most enginering.

I would also think that the closer to the head the turbo is, the faster the spool up.

The size of the exhaust after the turbo is from what I understand should be big as you can fit in there.