We seem to have wandered very far indeed from the lenses relevant to the original comparison, which were small, inexpensive kit zooms for Four Thirds and Micro Four Thirds! I am glad though that you are following my advice of using a smaller, higher resolution sensor instead of a TC with a f/5.6 lens!
I've been through this exercise before. In the absence of direct and competently executed comparisons, I find it difficult to make any decision when it comes to opting for a different system, or going to the expense of adding yet another system to what I currently use. I also think it's reasonable to suppose that the true reason why there are no comparisons between 4/3rds camera/lens combinations and Canon APS-C and FF systems (apart from Dpreview comparisons using a standard lens), is because there's nothing to shout about regarding 4/3rds' performance. In other words, you get what you pay for. If a particular 4/3rds camera and lens is lighter and cheaper than the Canon equivalent, it probably produces at least slightly worse image quality, on balance. If the performance is noticeably better than the Canon equivalent, the 4/3rds' system is likely to be more expensive and heavier.
I notice that Photozone have tested the Panasonic 14-45 & 45-200 with the G1. There's a very close Canon equivalent in terms of price, focal length range, maximum aperture, price and weight. It's the Canon 500D with EF-S 18-55 IS and EF-S 55-250 IS. Photozone have tested both of these EF-S lenses using the 50D which has the same pixel count as the new 500D. Photozone advises against comparing systems because of issues of AA filter strength, pixel count and choice of RAW converter. However, I think it would be reasonable to assume that the image quality from the 500D will be very similar to that of the 50D.
The Australian price of the G1 with both kit lenses is slightly greater than the kit price for the 500D with the two EF-S lenses, about A$50-100 greater comparing a range of internet prices.
On the other hand, the G1 with both lenses is 115gms lighter than the 500D with its two kit lenses. Neither $100 nor 100gms either way, is likely to influence my buying decision by itself.
Comparing the Photozone results for these 2 lenses using the G1, with the two EF-S lenses using the 50D, the Canon lenses, on balance, produce better results. There are a few intances where the EF-S lenses are significantly
sharper (the 55-250 at 55mm), and a couple of instances where the Panasonic lenses are significantly
sharper (at full aperture at 14mm and 18mm, but only in the centre). The EF-S 55-250 is sharper at all apertures and focal lengths tested, although not always significantly sharper. Both EF-S lenses are sharper at the edges at all apertures and focal lengths tested. The Panasonic lenses seem to have a fairly severe vignetting problem. At 200mm, edge performance is abysmal on the Lumix G 45-200, at all apertures tested
, including F11.
In my kit, my closest match for the 100-400/4-5.6 would be my 50-200/2.8-3.5 used with and without 1.4x TC, though effectively 2/3 stop slower.
You mean, the 50-200/2.8-3.5 with 2x converter, don't you?
But if you are comparing super-telephoto options for Four Thirds, you might want to check out the Sigma options too. The Sigma 70-200/2.8 with and without 1.4x or 2x TC's offers similar speed/FOV options to the 100-400/4-5.6. There are also some extreme options like a 50-500/4-6.3 for about US$1100, or a 300-800/5.6 for the price of a small car. What would be more interesting for me though would be if the Sigma 100-300/4 (about US$1100) were offered in Four Thirds mount. For now that is available for Four Thirds bodies only in a clumsy manual focus form, via the Nikon mount version used with a lens mount adaptor. (Never mind the need for stop-down metering; I would probably not be stopping down often with that "600mm, f/8 equivalent"!) To be more ambitious, the Sigma 120-300/2.8 (US$2,900) would be tempting; it is supposedly as sharp as the Sigma 300/2.8 prime, and about the same price and weight.
All comparisons I've seen between different lenses with and without converter, suggest that the shorter focal length with the converter is not as good as the longer focal length without converter. For example, the excellent and highly regarded Canon EF 70-200/F2.8 IS with 2x converter is not as good as the humble 100-400 IS at 400mm
I've come across lots of glowing reports of the Canon 300/2.8 IS, supposedly one of the best lenses that Canon produce. Yet I've never come across a direct comparison between the 300/2.8 IS with 1.4x extender and the 100-400 at 400mm. Why is that? Have I not been searching hard enough? Again, it's a reasonable assumption that the reason there are no comparisons is because there's nothing to shout about. The 300/2.8 with 1.4x extender is probably very marginally sharper in the centre than the 100-400 IS, and very marginally less sharp at the edges. No big deal either way. If you already own a 300/2.8 IS, then it's probably not worth getting a 100-400. However, if one already owns a 100-400, then going to the expense of getting a 300/2.8 in order to improve upon the 100-400, doesn't make much sense.
I'm really only interested in significant leaps in quality, such as that offered by the D700/14-24/2.8 combination compared with the 5D/Sigma 15-30.