If time lag for pricing adjustments is a factor against holding the sale in other stores, may I ask if there's any particular reason why you're not willing to offer the same pricing in your own direct-buy webstore, where presumably you control when the sale would start and end?
Otherwise, you end up preferentially skewing and biasing the test by cutting out the portion of the potential sales market which uses ePub and/or doesn't shop via Amazon.
I myself have a Kindle, but I consider Amazon's proprietary format to be technically inferior and really not worth paying for in most cases and I do not care at all for certain of their business practices, such as charging customers who happen to live in certain countries an extra $2 per Kindle title*.
As a result, I would not normally† spend my money on titles offered exclusively on sale there which were otherwise available elsewhere, no matter how low the price drops.
However, I'd be quite happy to buy direct at the same price from a publisher or author, who would presumably get more of the money involved in the sale, as I offered to the
last time one of these 99-cent Kindle-only Bova specials was advertised here on MR.
Which I passed on, because I never got an answer and as I reiterate, I view an Amazon-only sale as a sign telling me I can save money! 
Though possibly not in the way the promotional seller intends.
* You can see this for yourself by logging out of your Amazon account and switching your country/region in the storefront drop-down to Middle East (and/or Latin America, IIRC) and seeing how the prices change, which they also do for a number of Western European and other countries as well (but you'd have to enter full fake address info for that).
† I can count on the fingers of two hands the exact number of favourite authors whom I'd be willing to buy in Amazon's Mobi format even on deep discount sale and I wouldn't even have to use my thumbs.