Nozzle size? The effect of it?

I am running a chinese .4mm j-head clone with great results. I have nothing bad to say about the jhead design or the chinese clone. I also do not cool the jhead at all and it is mounted to an home made aluminum piece so the effector does not warp.

With that being said. I am thinking of trying different sized nozzles. I like the .4mm head. I will be buying an all metal j-head with three different nozzles (.3mm, .4mm, & .5mm) so I can test out the print speeds and the viability of using an all metal j-head.

Before I ask the important question. I will say the following.

1 - I only print in ABS
2 - for now I have no interest in printing in PLA at the present time.
3 - yes I use a heated bed (soon to have a pid controlled heated chamber)
4 - I am setup to use 1.75mm filament.

What is the largest nozzle orifice nozzle I can use? I have seen .6mm nozzles but no larger. Do they exist? I am not beyond buying two or 3 different .5mm nozzles and using a fine drill to step up their size. What are the downsides to using a larger nozzle?

I am aware that the resolution will somewhat be hindered by larger nozzles, however I am not sure what other things I need to take into account. Temperature? etc etc.

Educate me if you have any clues.

The goal for me is to have a .4-.5mm layer height so I can print larger parts at lower resolution faster.