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Firstly I'd like to add that even small amounts of water injection significantly change MBT.
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I think we are saying the same thing in different ways. Perhaps a difference in interpretation of certain words.
I don't think a 5 degree shift is all that big. I see a 3 -5 degree change in MBT as an expected change that is easily delt with.
That is why I included the following statement to periodically "tweak" the timing slightly.
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I would deal with timing as the last careful tweak to the step wise tuning process. As mentioned above your WI mixture will modify your burn speed and that in turn will slightly modify your MBT timing. So every so often you would double check to see your still near MBT, then go back to tuning via AFR, boost and WI mix.
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In the NACA report E5E18 ( 1945) it shows that the shift in MBT timing is roughly proportional to the amount of ADI fluid injected, and depends to some extent on the mixture ( ie alcohol/water mixes had less need for additional ignition advance that pure water did.) There were also some differences noted depending on the location of the ADI injection point. (probably due to differences in the mass fraction that evaporated pre-intake valve vs in the cylinder)
So your rule of thumb for tuning would be:
As you increase WI rate, you will need to add a small increment of timing from perhaps 2-5 degrees, for each major step in injection rate, (ie going from a 10% to a 20% rate.
In the above NACA study they saw a change in ignition timing for MBT going from 29 deg -to- about 41 deg advance when going from no injection to 1:1 fuel/ADI fluid, when the ADI mix was 50:50 water ethanol.
This test series was run in 1945 where many of the other studies were run from 1938 - 1942 time frame. I understand that the ignition timing was not readily adjusted on many of the aircraft engines, so early studies focused on fixed timing operations.
The important aspect of the issue to me, is that it should be one of the last means used to bring the tune in. Too many people use it as the first thing they modify to solve knock problems.
There are some people that just keep dialing in advance and do not realize the dangers of overadvance in a high performance engine as the cylinder pressures can go orbital with just a couple degrees of excess advance.
Fuel air ratios also effect optimum ignition timing due to changes in burn speed. So by adding WI rate, and leaning the AFR you are introducing opposing effects. In the same study they found spark advance for peak power plots as a bath tub curve ( ie shaped like a large U ) when plotted against fuel air ratio. Minimum advance occured near AFR's of .08 - .095 (12.5:1 -- 10.5:1) with rapid increases in necessary ignition advance either richer or leaner than those mixes. At an AFR of .06 ( 16.6:1) the ignition advance needed was about 14 degrees more than the minimum.
Due to the effects of WI rate, mix and AFR on MBT, you actually have about 4 different ways to change effective ignition timing.
Larry