I dont know if it applies to this power boost stuff but...I've known people to use this sort of thing and it to have had an adverse effect. The problems that you face are caused because the little cracks and holes that are in your engine are blocked up by carbon deposits and when a cleaning agent is used, it unblocks all the little holes that have built up over the years and causes problems that werent actually a problem before!
I personally have always used Redex, on my mates reccomendation. My mate has always used it, he used to have an E reg Nova and it worked fine for him in that. Cant say that I have noticed any improvments as I have used it since day one.
Another related additive you could use is octane booster, doesnt clean anything but gives your engine better fuel to start with.
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Wanted: Car with no rattles