You're as busy at work as I am Jools???
Been thinking down very similiar lines to your post. The options would appear to pan out like this:
851/888
You're probably going to end up with an 888 as the 851 option is always a bit nerve-racking. But you want the bike, primarily as a TD bike, so you'll always be holding back 10% if you're on a nice shiny, not easy to replace, 888. Wait until you can buy one for your living room!
ST2/4/4s
Too much weight. As you point out, a 1000DS is already 30kg lighter.
Carby SS
Cheap (£2k for a good base bike). Reliable, but a bugger to get real stomp out of. At silverstone I kept finding myself behind the same R1s/ZXs/etc for lap after lap because I never had enough stomp out of a corner to show a wheel. I'm thinking about 100bhp too but with extra grunt.
FI SS
If you like/can-live-with the looks then a 1000DS has to be the best option. Rumour has it that a 1080 kit and some tweaks see around 105bhp but with a major dollop of torque for those gixer-eating moments.
Put it on a diet and you'll be looking at 105bhp/175kg which are fine stats, but, and it's a massive but, they're rarer than hen's teeth and still fetching high prices £5.5k!
So there's no perfect option. The best, financially, is a £3.5k 916. Until it goes wrong.
The best for TDs has to be a really cheap 900SS. Ditch the lights/indicators/switchgear/heavy seat/rear pegs/heavy pipes/rubber hoses/fairings.
Put on pattern half fainings/velcro LED pushbike lights/hollow carbon cans/ rearsets/front dymag/wide bars, and ride it like your arse is on fire.....
Next year you can then buy a 1000DS lump from a breakers yard and spend £3k getting it sorted.
Cheers,
Ali