I don't see how ProRes would be huge since you can convert your H264 to ProRes (although I don't see what you would, ProRes is overrated).
If you think ProRes is overrated compared to H.264 as an acquisition codec I'd suggest you don't have enough experience with post production to make a judgement call.
It isn't a question of whether you can convert to ProRes or any other format later. Encoding the 14bit data off the sensor to ProRes preserves far more of the original information than h.264 - which is something you can't get back by transcoding the h.264 to ProRes later. It also introduces far fewer compression artifacts than h.264, which will allow you to maintain a cleaner image through multiple encoding generations from post to delivery. Encoding it in camera saves you both the time of a transcode later, and the generational loss that comes from that additional encode.
It does all of that while producing files that are significantly smaller than the raw data off the sensor, giving you longer record times per card and allowing you to use cheaper, slower cards. It's significantly more practical than working with raw, with far, far less compromise than h.264.
Has there been any test to prove that ProRes capturing gets you a better image than h264 ?
ProRes is a significantly higher quality codec than h.264 (especially as implemented in camera).
10 bit, 4:2:2, intraframe-only, low processing requirements, high bitrate but low enough to be manageable in post without exotic hardware. It also holds up extremely well to multiple generations of encoding in the post process; it's designed from the start for post-production.
H.264 is 8 bit, 4:2:0, interframe, low-bitrate - it's really a delivery codec that's been forced into the role of capture by camera manufacturers. It's also very computationally expensive. That's become less of an issue on the post production side due to GPU acceleration, but it's still a concern on the acquisition side as compromises have to be made between performance/heat/battery life in a camera body. Canon's DSLR implementation is particularly weak due to this, which is part of the reason there's so much to be gained from raw capture.
Just the 10 bit vs. 8 bit difference is huge for post production. That's 1024 levels per channel vs. only 256 - that especially becomes a significant advantage when you're shooting a flat or log-like color profile. In that situation with an 8-bit codec you're essentially sacrificing mid-range fidelity for shadow and highlight detail - which can negatively impact important things like skin tones. With 10 bit you get more levels each in highlights, midrange and shadows than you get in the entire 8-bit image. Now compound the issue by adding the 4:2:0 color sampling of h.264 and you are only making a bad situation worse.
Now you asked about a test - the problem is the test isn't as simple as recording the same image with both H.264 and ProRes and then looking at them. There are tests out there from several cameras (internal vs. an hdmi ProRes recorder), and they basically show only minor visual differences. That's exactly as expected - h.264 achieves it's high compression ratios by discarding information which is not visually noticeable, so it looks great to the eye. To really see the difference you need to shoot the same footage with both then run it through a full edit, color correction and encoding for output. The problem is that the information h.264 throws away - which is not visually noticeable - is often useful later in the post process, and can become visible later in your finished piece.
I don't think we're likely to ever see ProRes available through something like Magic Lantern. Enabling raw recording is a process of elimination - skip the on-camera scaling, debayering and compression steps and just write the data to the card. ProRes or another high-quality codec would require adding new encoding code to the camera, which is a much more difficult process. I think it's especially unlikely with ProRes because there's no open source implementation of the codec; if they do add any additional codecs they're likely to be something that's non-proprietary.
My main interest in the blackmagic cameras though is due to their ability to record straight to ProRes. I see raw as a bonus that you might use in specific situations where it could give you an edge, but direct to ProRes is the everyday shooting format that still outperforms all the other compressed camera formats available now.