Hey mate
I have a Carradice Barley on my commuter and love it... I leave it on all the time. As a climber who lives somewhere flat and VERY windy I think it tucks out of the wind much better than a rack bag would. Just be aware it can 'sway' when out of the saddle, tho I don't mind it.
I think your seat is high enough for the bag not to reach your guards, tho you should probably use a support (Carradice make one called the 'Sport Bag Support'). The support will help preserve the straps, prevent the bag from bumping yr legs as you ride, reduces sway, and can 'make the bag hang nicer'
Just be aware the 'supply' of this support varies (seemed to be unavailable for quite a while last year via web orders from Oz).
For security you could carry all yr goodies in another 'hand bag' inside yr Carradice and simply remove that when you leave your bike unattended (as a friend of mine does), this also doubles as 'inner pockets'.
I also leave my '32 quid' pump inside my bag, I have attached it to the wooden support with 2 thin strips of inner tube and 'tuck it under' the support so that prying eyes don't notice it is there.
I seem to remember your saddle has the correct bag supports.
Just be aware that you entering a slippery slope here... First a Carradice, then a Brooks, then a Son, etc... etc...