If you upgrade your core, it would be wise/most noticable to upgrade everything. Bigger lines, better pump, bigger FMHE. A local went from an SSIC to a S2 IC and noticed very little gain since he left everything from the SSIC installed and just used the S2 core.
Here is the FMHE I will be using. A few people on clubgp have made it fit without removing the A/C and from what I hear I will have an easier time since there is more room in Regals to work with.
eBay Motors: VW GOLF JETTA SCIROCCO RADIATOR 84-92 ALUMINUM CORE NEW (item 250222363581 end time Mar-11-08 13:48:40 PDT)