200 gm/sqm either side of 2mm coremat may be a “light” finished laminate but it will also be a very flexible laminate with fairly low impact strength and also require extensive internal "framing" for rigidity integrity over any reasonable square area, which could negate any weight savings if/when going to such a light lay-up. Two laminates of 200 gm/sqm CSM plus 2mm coremat plus resin would equal approx’ as low as 1.2kg/sqm to 1.5kg/sqm depending on how well the resin (polyester) is distributed (these weights assume that the person laying up these laminates is able to remove all the excess resin, if not then the weight will regularly result to be approx’ 30% plus higher if “amateur” built due to over resining) PLUS the extra weight for any additional reinforcing framing. The better glass to use for a Coremat laminate would seem to be CSM rather than any woven product, which in practice delaminates much easier than the CSM with impact and/or excessive flexing.
For two laminates of 300gm/sq CSM plus 2mm of coremat plus resin equals approx’ 1.8kgs to 2kgs/sqm.
With the heavier laminate, over 60% of any internal framing can be accomplished using “bare” polystyrene, epoxy resined or siliconed into place, but with the lighter laminate then approx’ 90% of internal framing would require rigid (heavier) framing, IE marine ply or better still, semi rigid PVC foam sandwich constructed “frames” and bulkheads – six of one, half a dozen of the other?