Ok, so it looks like I missed where you said you have a stock pulley, because you just kept saying underdriven. I don't consider the stock .95:1 pulley underdriven because it's a stock pulley. Yes, I understand that technically it is underdriven. But when you kept saying underdriven I assumed you had an aftermarket underdrive pulley and that's a whole different thing. That's on me.
You also said you didn't know what your water pump was. The terminology also gets confusing, because the factory used "standard" for the 8 blade pump and "AC" for the 6. Well, if you go to buy one now the 8 blade pumps are "high flow" pumps, not standard.
So, sorry for the misunderstanding. I do run a 1:1 pulley because I do have an aftermarket pulley. I wouldn't worry about the 5%, but you do need the water pump flow capabilities matched to the pulley drive ratio. If you have a .95:1 pulley and an 8 bladed pump then you do have the correct ratio, but that's not how I understood your posts.
Well that's terrible advice. Would you like to name name's and list the "industry people" you spoke to so we can clarify that overgeneralization? Because you absolutely have to pay attention to pump performance and pulley ratio's.
The number of vanes on the impeller isn't a good way to measure the pump performance, I'll give you that. But most of the pumps we are talking about aren't rated in GPM and the OE pumps can be separated by the number of vanes so that's commonly what's used. Now, maybe your industry people have the actual GPM numbers and therefore wouldn't use the number of vanes to categorize pump performance because GPM is the better way to do that, but saying it doesn't matter isn't accurate.
And just overdriving to the maximum and running the highest flow water pump isn't necessarily the best solution. The OP is talking about running his engine over 6k. Why do you assume efficiency had anything do do with the 1.4:1 ratio pulley being used with a 6 vane OE pump instead of an 8? Because most of those pulley ratio's and water pump specs were pre-'70, and efficiency regulations weren't really a thing then.
Running a massive overdrive on a high flow pump is not what you want to do, especially at 6k+.