Don't hold your breath for these results in/near London
For both myself and my colleagues with data tariffs on our phones we've found Vodafone data performance to be shockingly bad for an awful long time, and calls to customer support provide no signs of any improvements. It's certainly much worse than for colleagues on O2 and T-Mobile.
As to the dongle, the consultants from one company who come on our site use these, and all of them (3 people) have problems with connections dropping regularly for no apparent reason while getting supposedly decent reception. Their data performance matches our experiences on out phones.
The fact that in Central London we get really poor data performance from Vodafone has always put me off using one of their dongles, and I've even considered jumping provider ship.