I've been using a 135L +1.4xII for stage performances lately with great success... not to hijack the thread, but would adding an extension tube to it allow for a usable macro combo?
Adding my two cents.....
I am mainly using the excellent EF100/2.8L IS USM Macro, (bokeh/versatile/excellent IQ) but also the EF 135mm f/2L sometimes with the ExtensionTube ET25II and sometimes with both ET25II+ET12II.
The latest addition will be the the Canon 500D Close-up Lens, to be used together with the Canon EF 135mm f/2L, ( but I am still void (I have placed and order coming soon) of a step-ring-adapter to use the 77mm Canon 500D (using it on my EF 70-200mm f/2.8L II, because, if you are carrying a non-macro lens, and you want macro capabilities, without the added weight and size of a macro lens, the Canon Close-up Lens 500D could maybe be a perfect option for you in these cases?
Anyhow - the quality of the pictures taken for example with the Canon 100mm Macro Lens I suppose will be even better compared pictures using the Canon 500D Close-up filter.
I have not used the excellent lens of Canon EF 180mm f/3.5L Macro.
The closest long-focal-lenght-macro-lens that I have used is my excellent yet non-L-lens the old Canon FDn 200mm f/4, together with an adapter FD to EOS, on my 5DMkII during the last spring season, and I like what that lens can do!
However having the EF 50/2.5 with the Life-sizer, the EF100/2.8L IS USM Macro, the EF 135/2L and also the EF300/4L IS USM and in addition the both Extenders III//Extension Tubes II, I do not see the real benefit of even adding the great Canon EF180/3.5L Macro lens to my lens collection today! (The 180mm is quite heavy and you need to carry of lot outdoors if you also have this lens in your backpack).
But if I would not have any of macro lenses from Canon, I still think that my first choice, would most likely be the new Canon EF 100mm f/2.8L IS USM Macro.
This lens is an extremely fun lens to use! It combines excellent image quality with a useful focal length, a relatively small/light body, fast/quiet AF with 1:1 (1x) macro capabilities, - and HIS (Hybrid Image Stabilization).
But it depends what you prefer to photographing!
However the Sigma 180/2.8 might be interesting but it is quite heavy (3.6 lb (1.63 kg)).
It would be nice to test this lens because of its abilities to create a beautiful bokeh!
Wish you sir the best choice that will suite your needs all the way!
Greetings from Sweden,
Adding one photo captured with 5DMK3 + EF100mm f/2.8L IS USM handheld with f/2.8 ISO640/handheld resting the camera body on the ground.