The Valver doesnt make much more torque that the RSi admittedly and it will be at higher revs, but it will still be a quicker car than the 2.0 Laguna lump. Plus, you cant swap the bottom end from a 2.0 8v Laguna AFAIK.
Then, of course, there is the Willy lump. More than enough torque at low revs to kick the Laguna 2.0 into touch and, of course, more power than the Valvers F7P.
Personally, unless it was really cheap to do any of the above to the RSi (like well under £500) - then youd be best off selling and using a Valver as the starting point. Just think of the chassis and suspension benefits of a 16V for starters.