I have removed a couple of redundant deck fittings, I was planning on filling the holes, after cutting a small flange, with epoxy. Would it make any difference if I used a small amount of low density filler or similar?
I'd always add coloidall sillica to any epoxy used for filling/fairing. According to my very knowledgable chum (ex Green Marine, Fairline, Halmatic et al) The sillica stabilises the resin/catalyst mix otherwise the catalyst tends to penetrate the laminate leaving the resin behind.
It is also a darn sight easier to handle as "mayonaisse" than "golden syrup". For a basic fill and fair job I suggest a mixture of sillica and low density filler.
In fact solid epoxy tends to crack. Usual boatbuilders dodge is plug hole with Sikaflex up to about 3 mm then finish with gel coat. Being flexible it never leaks and it's only drawback is that it discolours under UV light. A good chamfer on the hole will be OK for bonding gel coat and if you use a fine plastic scraper to wipe over the top you should not even have to do more than polish it
Microporus ballons are very good for adding to epoxy where you want to sand it back. Coloidall sillica is used to thicken and increase the adhesive qualites of epoxy. Microporus ballon filled epoxy is strong but easier to sand than epoxy with colloidall silica.