We just installed the block, which is made of aluminum, on the strut with SS screws beaded in Tefgel, no problems as yet and they have been there for 19 years.

Withe regards to isolation we frequently attach stainless to Al P brackets, they are protected in the same way.
I have an article about cathodic protection I can e-mail if anyone is interested this is the part about quicKutters.
Isolating is not always the answer as they are all in a conducting electrolyte.
quicKutters are constructed of 316L stainless steel and therefore may be
regarded as cathodic, in much the same way that a stainless steel
propeller shaft is cathodic to a bronze propeller. The entire submerged
metal componentry of a vessel should in any event always be electrically
connected to sacricial zinc (or, in some cases magnesium) anodes which
should be strategically located, constantly monitored and replaced
regularly.
quicKutters are cathodic to marine structural and propulsive metals that
comprise a vessel such as steel, aluminium, and bronze. Should a
quicKutter be fitted to a vessel, the additional mass of cathodic metal
componentry compared to pre-existing componentry such as stainless
rudder and prop shafts, bronze or stainless propellers, or brackets, is
relatively insignificant.

Just to clarify, I did not say that Spurs were the best, but simply that we have used them for a long time with good success. At the time we installed them, that was all there was. Things may have changed, but we have not done any sort of evaluation.

As the post above says, we installed the Spurs and have been using them now for about 25 years on two different boats.

We have never had any problems with the SS screws into our aluminum shaft strut. Although it is important to read the note on isolation in the post above.

Motor Boats Monthly did a test of various types and a nice video is located here: http://www.youtube.com/watch?v=JImiuemabIY

Having looked into the various ropecutters the disc type appears the easiest ones to attach and maintain, but the knife like cutters the most efficient. However, I really dislike the idea of drilling into an aluminium the stern tube to attach a stainless item with stainless screws. Views and/or comments on this objection or related issues are as always appreicated.

This cutter (quicKutter) is very different in the way it operates from the disc or scissor types. It is also the one used by the UK RNLI and the Royal Navy.

We were asked to participate in the YM trial you mentioned but had to decline for two reasons. Their test rig was the one made for the gator cutter, a scissor cutter. This requires a large gap between the prop and the bearing carrier and the test methodology was designed to show how the cutters could cut and not what happens if debris is caught by a prop blade. I was there at the test.

To get these to cut rope they fixed one end of the rope to the P bracket so it would be dragged along the shaft close to the cutter, this also meant the debris was fixed relative to the rotating shaft. This could be the reason why the discs seemd to work so well.

The quicKutter cuts debris that is caught by the prop and hence rotating and winding on the shaft, this is after all what happens in practice if you look at the result of catching a line or rope. It winds in the gap until it welds into a solid lump or in worst case pulls the gearbox off the engine. The shaver cutter (quicKutter) is a fixed blade angled so that it shaves into the debris as a lathe tool would.

We have many fixed to al carriers and to date no damaged fixings, partly as there no shock loads when debris is cut.
You can see how it works better here.
http://www.youtube.com/watch?v=Qh9SnDzXJXk