Take care when putting SP (L) heads on R (H) cylinders as this raises the comp to a point where you'll need good fuel.

If you've got SP/SE (L) cylinders (taller than H) then fitting the R (H) head will lower the comp. Not too big a problem if you're just cruising through town, but it may make it harder to start and will lose bottom and midrange power. Top end will be a fraction down, but with more over rev.

Raising comp has a very similar effect to advancing the ignition.