Lots of things to consider. The advantage of an IC upgrade depends on how you drive, where you drive, load you are carrying / towing, etc. In most cases the advantage gained is minimal - especially when you consider cost and complexity of some installations. I went for a Top Mount aftermarket IC (only because my crappy original Nissan one leaked) and had lots of problems initially with it fitting properly - among other issues.
If I had to do it all again I would have taken the original IC and had a more efficient core TIG welded onto the existing ends. Good value for money and everything fits as it should - and it is easy to do.