Here's the other side of the argument - I recieved this mail from David Wilks who invented Interflush:
As you are asking about the interflush, I thought I had better enter the conversation, since I invented it. There is really no contest with the Interflush versus anything. It is dead simple. The siphon is the ONLY flushing device which never leaks, its proper name being 'the Water Waste Preventer' because it replaced leaking valves back in the 1860's. A siphon stops flushing when air is allowed to enter, hence the various plugs for reduced cistern capacities. All the Interflush does: is to introduce an air valve on top of the EXISTING siphon, which is closed when the flush handle is down (allows flushing) and open when the flush handle is up. Press, hold down to flush, let go to stop as soon as pan is clear. Just by simple logic, NOTHING can flush a toilet with less water.
As for changing the pan to get a lower flush. The rated flush of a pan is largely determined by the volume and hence the mass of water in the bottom of the pan, the water spot'. Reduce this and the less water is required to push it round the 'U' bend. The difference between pushing a large boulder and a small boulder.
Now the energy consideration. Ceramic ware is in a kiln for 17 hours at 1400 C, with a consequent large energy use and carbon emissions. The embodied energy of the Interflush is less than 1Kwh. No contest. Purchase of such a low flush pan is a good idea for new build but for renovation work or water saving, it is folly to throw out a perfectly good toilet pan. Just fit an Interflush and cut its water consumption by 47%, which effectively turns an old 9 litre pan into a 4.5 litre pan, all for £20, 1Kwh and about 15minutes fitting time.
You can also fit the Interflush to siphons in single flushing low flush toilets and save a further 47%. The whole point to the Interflush and Interruptible flushing is that you just flush what you need to clear the pan, the notion of flushing FIXED VOLUMES of water becomes an obsolete idea as some is always bound to be wasted. David Wilks www.interflush.co.uk