With much anticipation I loaded the V3 firmware but the sensor is cropped to 46% and 25p is limited to 100fps - not what I was hoping for.

Ah, so thats why they added 46%, how is the detail, is that holding up?

Have only done a couple of very quick tests - still a lot more to do.
The detail seems about the same as normal speed with the sensor set to 46% - if you set the sensor to 86% and zoom the lens in to match the framing there is a very noticeable difference in sharpness.

Feeling a bit underwhelmed by this new feature - will probably stick to using the GH4 for slo-mo at the moment.

I have rented a HM200 this weekend for testing.
I agree. The slow mo feature on the HM200 works the same way. It is a little soft and crops the image.

Also, the slo-mo recording is limited to the 4:2:0 colour space codec. Better than nothing I guess...

A couple of other observations,
When you set the camera to Hi-Speed the shutter defaults to 360 degrees - not an ideal setting when trying to capture fast moving subjects. I assume they're trying to compensate for the loss of light that is inherent with hi speed shooting.

Even with the shutter set to 180 I'm finding the softness/lack of detail annoying.

Pre-record function is not available in conjunction with Hi-Speed - a shame as it can cut out a lot of useless footage.

I do like the ability to assign variable gain to the iris wheel - something I hadn't really thought about but I can now see myself using quite a bit for fine tuning. Have set assign switch 9 to toggle between iris & gain, I use to have it set as a record button.

I do like the ability to assign variable gain to the iris wheel

I think I was the only one requesting for that option and I couldn't understand why it was not more requested because how else do you finetune your exposure when you want to shoot at a particular f-stop? The slo-mo addition however was one of my lowest priorities, while I can understand there is some use for it I cannot think of any scenario that would require me shooting at those framerates for the kind of work I do. The only time I use slo-mo is for my trailers or for some of my steadicam work and then slowing down 50p footage 50% is plenty slow enough.

When you set the camera to Hi-Speed the shutter defaults to 360 degrees - not an ideal setting when trying to capture fast moving subjects.
At 120fps, a 360-degree shutter is 1/120 sec, exactly what I'd use for footage I intended to use for slow motion. What's visible in terms of motion blur is not the recorded shutter speed, it's the effective shutter speed at playback.

What's visible in terms of motion blur is not the recorded shutter speed, it's the effective shutter speed at playback.

Interesting - more testing needed.
I've always tried to shoot Hi-speed with 180 or less if light & camera allowed.