Many years ago we built some 12-14' skiffs with extruded polystyrene core.   They worked pretty well, but one of them had some major delamination which proved to be caused by paint solvents passing through pin holes and dissolving the core.   They also suffered a lot in high impact areas.  The light skins would deflect the core, which would not spring back fast enough so the skin would debond.  We also use polystyrene for the impact absorbing bows on most harrys.  With a layer or 2 of glass over it, depending on the size and density of the foam), it works well.  

The problems are the low impact, shear and compression strength and modulus so it can't be used in high load areas (between the rudders on the lee hull or anywhere that is walked on or sees high local loads, like the hull bottoms), which does not leave much.  

The greasy surface is hard to bond to, but a hot wired surface is not much better as it leaves a smooth(ish) surface with none of the partially open bubbles that increase the bonding area. 

I agree that H80 is costly.  Some savings can be made by using it in squares with say 50mm/2" spaces between them where the skins are joined.  This is done on the mega cats and tris to reduce the core sheer loads.  On the cargo/ferry, we are looking at solid laminates (as the survey requirements are so heavy that core is not really helpful) with stringers where required.  These can by laid up over polystyrene in the infusion as the core is only a former, or built in a mould and glued in.  Both these result in a non flat inside skin which is not ideal in living areas.   A bit more work and weight, you can also use timber or glass stringers and fill the spaces between them with polystyrene and glass over the lot. 

Infusion resin does not travel far across compacted fibreglass.  Infusion medium (similar to shade cloth) over the top of the laminate is one solution, another is infusion mediums which remain part of the laminate (eg Soric,) and add some thickness but also weight.  the former and the resin it absorbs gets peeled off and thrown away.  Intelligent Infusion uses small grooves in the foam to distribute the resin.    Adds a bit of weight, but uses less resin than either of the others.  All vac bag and infusion jobs must have holed in the core to let the air out and the resin in.  These are usually 1-2mm diameter at 50-100mm centres. The grooves and holes  can be machine cut and drilled, at a price, or it is easy enough, though tedious, to do it yourself.   

600 gsm per side of 20mm foam is a common standard hull laminate for 12m/40' cats, the stressed parts of 40' harrys and the unstressed parts of the bigger boats.  600 is about as light as you can go for areas which will be heavily trafficked.