GI caustics are a nightmare.. the only way to get them clean is ..overkill gi settings.. especially with reflective gi caustics. thats why its usually off.
id find an area where you have lots of reflective gi caustics, set it to BF /lc and do region renders upping the quality until the lighting /caustics look fine
then sit back and render the bad boy on as many machines as you can.
obviously imap/lc will be way faster, and when doing images that size i usually go for min of -10 and say -2 or -1 for max.. you still want the small details preserved after all..
however ive never seen imap deal with reflective gi caustics properly.. just end up with a few bright splotches across the image.
wrt rendering the imap at a lower res first.. i never understood where this idea came from.. afaik you may as well just use a lower min and max. it has the same effect.