Right, back home so here's the story:
Start: Honow - 1019hrs
End: Rudow - 1753hrs
Time: 07:34:50
So just a tantalising 1m23s off Gasman's time.
All in all though, a very good and enjoyable day from start to end, with the usual collection of highs and lows
General Highs
Very good connections 95% of the time
Bus stops directly outside stations
Catching two or three trains that I thought I'd missed
A couple of buses/trains that weren't on my timetable
General Lows
A couple of bus stops nowhere near their "parent" stations
A couple of buses that just refused to turn up
Not "giving it the beans" in a couple of situations where I ended up missing a train by seconds
A couple of poor door/carriage choices which gave me some nasty full-platform-length runs
...and a little more specifically
Low Losing about 10mins having made the wrong choice in a situation where I could double-back or take a bus
High Gaining some of the lost time through an untimetabled train
Low Missing a train by about 5s by not "giving it the beans"
High An unscheduled bus saving the day
Low Missing a train because of said bus dropping me miles from the station and having to cross four sets of very unhelpful traffic lights!
Low Realising that the route I'd planned (from a couple of years back) didn't cater for the U55
High Working out that I didn't have to adjust my route to fit the U55 in, and that I hadn't reached the crucial point yet anyway
Low Losing about 10mins by choosing the wrong end of the U55 to start at!
Low Sprinting off a bus for about 100yds in the wrong direction, then missing the train I needed by about 10s
Low Going in to the last change of my route with the indicator board on the stairs telling me I'd missed my train (and the record at this point)
High Coming down the stairs to see that the train was still at the platform and deciding to give it the beans
Low A very fat, ugly German coming the other way deciding to, very deliberately, step into my path
High A very kind German holding the doors of said train for me
Low Three unexplained, unnecessary stops of 30 seconds to one minute apiece on the last train, which (even if everything else had still happened) single-handedly cost me the record!!
Adding it all up, I'd say that mistakes on my part cost me about 30mins overall, so I think that sub-7 is certainly possible on the pre-U55 route, and possibly on today's route too.
As an aside, I think I can say I notionally beat the old record too, seeing as adding in the U55 cost me about 25mins thus:
U6 -> Friedrichstrasse
Friedrichstrasse -> Brandenburger Tor
Brandenburger Tor -> Hauptbahnhof
Hauptbahnhof -> Friedrichstrasse