A bit of an update.
Seems a little bit of an anti-climax (for moi) after hours of research but....

The rotors in the IHIS105 and it's bigger brother....the IHIxxxx (still don't know the designation - help appreciated) are different;
- at 183mm v 223mm or about 22% (infered from the pictures previously but removing the clearance for the bearing journals). Given my error in measurement, they may have made it even simpler and made them 7" (178mm) and 9" (229mm) which would point to a 29% length increase. However.....
- they also have different a helix setup. Strangely enough I've established they both go for a rotor twist along the complete length of the female rotor (5 lobe) of 180deg. More than likely a predetermined key performance sweet spot for IHI, and
- the rotors seem to be of equivalent diamater in each unit - around 127mm for the high speed 3 lobe and 105mm for the 5 lobe. Again this may possibly be simplified to imperial dimensions by rounding to 5" (127mm) and 4" (102mm).
- They seem similar, but I'm yet to confirm if the lobe profiles are the same - I suspect so, they appear to be from all the picks I've reviewed.

The interesting thing here is that the female rotor in each still only does half a twist (180deg), just over a longer length. So given the rotors are effectively rotating at the same speeds in each unit and the rotors are similar diameters, the added capacity is driven by the rotor length and I guess it's ability to draw/screw more air, rather than capture/bite more air.

- as mentioned before, the teflon, high speed, 3 lobe rotor is on different sides for each. I've established this is because the female 5 lobe rotor is to be the driven rotor in each case.
I did read somewhere that one rotor likes to drive and the other gets driven by the air.Will add if I find the link again.

- another thing of note is the large difference in the discharge ports, the SL55 unit being almost twice the area as the S105. You can also see the different rotor pitch in the "silencer plates" and how they are cut at different angles. I did find a port job done on a C32 SC over at the MB forums, to remove some of the material, but I'm not sure of the benefits.

- from all this deduction - if you look at tip speeds of the 3 lobe rotor, at 127mm and a stock SL55 at 23,813rpm you get 158m/s or 520ftps. If you crank it to ~17,000rpm with a 65/178 stacked arrangement on a SLK32/SRT6/C32 - the 3 lober is getting to 28,000+ territory and you're hitting the 617ftps, 423mph or 188m/s.....I guess speed is a relative thing.

I could add some pictures...will see.