They seem to run faster as a matter if course. When I travelled we were 40 minutes early at Euston despite a 30 minute delay at Carstairs.
It really doesn't help the ride and I made the point in both surveys I was asked to fill in that an on time arrival with less disrupted sleep is more important to me than arriving early. I really don't know why they don't just tell the driver to keep to time rather than continually run faster than is necessary, it can't be that difficult surely?
The Class 92s which haul the Mk5s have a max speed of 87 mph - that is hardly fast for both stock and a route that is cleared for up to 110 mph (non-tilt). The rough riding some have reported from the Mk5s vs Mk3s is due to the stock, not how fast they are being driven.
Generally speaking the Sleepers will be driven up to the designed service speed of 80 mph, with 87 mph (which is hardly a massive increase) being used if running behind time.
As Mag_Seven has explained, the Lowlander has a lot of slack in the timings on the southern WCML and this is why it often arrives c.30 mins early, not because the drivers are haring down the WCML to get into Euston really early. They're actually far more pushed for time on the Highlander which is fitting in around the peak traffic - this often arrives bang on time, but has had to go up to 87 (or 100 with a Skoda) to keep its slot amongst the Pendos, Voyagers and units.
So the train that arrives early you perceive has rushed at the detriment of passenger comfort is usually being driven at normal service speed and can accelerate/brake more steadily, and the one that arrives on time and you may perceive has been "kept to time" is the one that's had to be driven as fast as is permitted.
The majority of the Sleeper drivers are highly experienced railwaymen who have driven Class 1 passenger trains since you and I were in short trousers. They are fully aware they have sleeping passengers on board and are continually balancing passenger comfort with making appropriate progress on a busy railway.