In a nutshell:
Going to a larger FMIC will not significantly increase lag in the RA since, if you look carefully, you'll notice that the air is traveling the same distance. It matter in Subarus, when people would go from top mount to front mount, significantly increasing the distance the air had to travel, but in our case, the air is already traveling that far.
Metal pipes have an advantage over rubber or plastic pipes in that they do not expand as much. Everything expands, but when you are comparing millimeters (rubber/plastic) to to microns (metal), it is a huge difference.
The primary reason for upgrading to hard IC piping is because of the increase in spool up. There really is no down side other than weight. Personally, I would go with aluminum, but I know a lot of people go with steel for some reason.
Moving from a small single-scroll turbo (think RA) to a medium twin-scroll turbo (think EVO) will not increase the turbo lag. Going from a medium twin-scroll turbo (EVO X) to a larger twin-scroll turbo (Blouch Dominator, Garrett series, FP Red or Green) WILL increase turbo lag.