Micro 4/3s, my next step Macro strategy.

Please do not worry about the 100-300 having adequate quality because it does.  The Micro 4/3 cameras should do every bit as well as your 620 since their sensors are the same size.  I had an E-620 but now have the GX1 and GH2 with a range of Panasonic lenses.  As long as you choose the right lens for the job you should have no problems at all.  Now, having said all of that, I also have an Olympus 35 Macro, f3.5 and adapter which is a wonderful lens.