Thanks, glad you liked it
Yes DOF is shallow with macro and gets even shallower with more magnification and so does working distance.
I agree with macro junkie, a set of Kenko tubes (68mm) will give you 2:1 magnification (with the Canon 100mm f/2.8 lens) with no reduction in sharpness (unlike a filter or TC). In my opinion its the best low cost way to achieve more magnification.
As for macro filters I haven't used them since my film days and currently don't have or recommend them (or want them), so sorry as I can't add that.