Sorry man but I have to say this...
Being able to overclock something past its design hashrate is great, but that extra hashrate comes at a cost. Having the fans run constantly and the hardware running harder adds some risk to the investment. You're expecting that extra hashrate to equal an increase in value, which is absolutely true, but the extra value shouldn't be proportionate to the original $/Gh basis that you're calculating the value on. If your buyer is making extra BTC by running it harder, that harder risk should be rewarded. Maybe increasing the value by 50% on extra hashrate rather than 100% would make more sense, at least to me anyway.
Another example of this would be the eruptor blades. They shouldn't be based on a 13gh/s value, but rather a 10gh/s with an overclocking incentive.
This is just my opinion, if you don't agree with me then just ignore my post.