Here is my guesses, and the reasons for them... I know nothing...

| So what are the downsides to low viscosity?

It is runnier. Traditional wet layup needed a balanced viscosity that would wet out glass and the foam under it, give up bubbles, but not just run right off the part and onto the floor.

| Higher price?
| Lower strength?
| More toxic?

Nope. It is really just a design choice for the epoxy maker. There is a chemist fellow on boatdesign who hints at how to buy bulk epoxy from hexion and blend in the correct additives for a marine infusion epoxy. He suggests a 25$ a gallon figure.

| What makes normal epoxy thicker, and these special infusion versions thinner?

I am not a chemist. But I understand that a blend of Bisphenol A and Bisphenol F has a very low viscosity. Lower than either one.

And, since you asked, look at this link.
http://www.hexion.com/en-US/chemistry/epoxy-resins-curing-agents-modifiers/epoxy-starting-formulations

Seems really easy to make your own epoxy if the manufacturer give you a recipe and sells you the material at the bulk price. Or one could have someone make it for you.

I don't know if this is practical. And it feels like a trap. Experimental foam, blend your own epoxy... I'll be extruding my own glass and weaving it next!