Heathrow Hub have long said their option can handle the same amount of traffic as Heathrow's own third runway (
Press Releases : Heathrow Hub - third article as of 28/9/16).
With regards to safety, operating what is two runways that are in line with each other is of course going to be riskier than operating parallel runways, however some would say loading 400 people onto a 400 tonne aircraft filled up with kerosene is risky.
In terms of aircraft go-arounds, the Heathrow Hub proposal is actually safer than operating from one runway as at Gatwick and many other airports across the UK. The risk I assume Trash 'n' Navs is referring to would be an aircraft over running from the landing runway onto the take off runway. Assuming the aircraft left the landing runway at 100mph and that the takeoff runway starts 650m after the landing runway (what Heathrow Hub have claimed the safety gap would be), it would take approximately 15 seconds to travel the "safety gap". Given how closely monitored LHR's runways currently are - landing aircraft are given clearance to land only when the aircraft in front has cleared the runway, which is often only seconds before the next aircraft touches down - the controllers would certainly notice an aircraft shooting along the landing runway towards the take off runway. Add in the fact that the safety gap may have some system in place to stop an aircraft that enters the safety gap and the chances of the landing aircraft reaching the takeoff runway whilst an aircraft is still there are very small.
As for the airspace redesign, whichever runway option is chosen (assuming any are) the airspace will have to be redesigned regardless. Even without any new runways, given the increasing pressure from communities around airports for respite, an airspace redesign would happen sooner or later.
DfT clearly believes the Heathrow Hub is a safe option otherwise they wouldn't be asking Heathrow Ltd. if they would be prepared to implement it.