Water down the carby shouldn't do any harm to the motor or the carby, but again I stress that you do things in moderation. When I've done this on my old XY, I get the hose going at a very small continuous dribble, with the stream no thicker than, say, a drinking straw. But the motor has to be running, and like I said previously, you may need to manually rev the motor to keep the rev's up.
Now if this method clears up your problem, then you have a couple of possible scenarios to consider. Either the carbon had built up previously and now that its gone, the correct tune should maintain things and prevent it coming back. If the problem re-appears after some time, then you can probably assume carbon has built up again and just repeat the treatment OR as Addo mentioned, install a permanent water injection system to keep the carbon at bay permanently.
If the treatment doesn't get rid of the problem, it may not be a pre-ignition problem from carbon. As I think someone else mentioned on another related post, have you considered the possibility of this being detonation, rather than pre-ignition?? If it is detonation, then installing a permanent water injection should help in that department. Seeing as you're running 10.5:1 compression, detonation might be a possibility.
Another thought that just popped into my head...you said earlier that it still bogged down a bit even after changing from the 600 Holley to the 450... are you sure that the 450 isn't still too much carb? I know that a lot of other people are running 350 Holley's which gives them good drivability for their application. Think about what your normal rev range is for general driving conditions. Changing diff ratio will merely bring up the revs, thereby bringing the larger carb into its prefered operation range for air flow. You be the judge as to whether its cheaper to change a carb or change a gear ratio.
That'll give you a few things to think about, so I'll leave it there.
Regards,
Teddy