Hi Tom, as far as I am aware the signal stacker option saves on extra internal wiring. Using this option will combine the existing feed with new extra feed that is being installed. Prior to the upgrade work the only option was to hope you had a spare feed in your riser cupboard. This work will mean all risers have spare feeds. The stacker option means less work as no extra flatside internal wiring is necessary.
This work is not specific to HD as it has always been possible to get an HD signal if you have the equipment. There is no issue with regards to leaving this until another date. I was just putting it out there so if there were people who were double keen to get multi channel recording up and running we could sort it out sonner rather than later.
As an aside I recently went down the Freesat route and I am quite suprised just how many multichannel recording options there are despite only having one sat feed.
Hope that helps.