Ah yes, the old Top Speed debate.
For me personally the Top Speed doesn't matter anymore. I've driven my M5 past 200mph 3 times, and although its exhilarating, now that I've done it, I'd rather have better performance down low in the speed range which is what the E60 M5 is not good at. I've only had a handful of times when I have actually used the E60 M5's superior high speed performance to defeat rivals, so there's a testament to the limit of its usefulness.
Moreover, as gas prices have gone up here in the U.S., especially in California (where gas is easily $4.60 per gallon for crappy 91 octane), I'm ready to trade the thirsty V10 in for the F10 M5's twin turbo V8. Last night I filled up my E60 M5 and paid over $74.00 USD, which is insane if you have to fill up your M5 every 3 or 4 days.
Me personally, stock for stock, I think the E60 will catch up to the F10 around 160 - 170 mph.
The real question is, since about a few dozen people have taken delivery of the F10 M5 in the United Kingdom already, why hasn't anyone over there run the E60 M5 against the F10 M5 to test this top speed debate?!?!?!?! Not sure about the speeding laws over there in the UK, but here in America, we would have found out the answer to this debate if the F10 had been delivered to the USA already.
So stop hypothesizing UK owners and produce some results already! Or else I'll sick my alligator on you!