Got a prophet and thinking of upgrading the float r to something better and have read a few things on t'net about the effect of stroke changes reducing travel/adding travel etc. Assuming it works and doesn't cause frame contact between shock mount or tyre will the only negative be raising the bottom bracket and slackening the head angle?

You're going to a longer shock? That'll steepen the head angle and the seat tube angle, dependent on how much more sag you run.

You'll void your warranty.

sorry I did mean steepen my bad, I mean just a longer stroke shock so the swingard travels further up/in. So I guess this would not affect geometry only travel (and if the wheel hits the seat tube) though would probably affect geometry when sagged.
Don't have a warranty, probably never will being a tight sod and all.

If you got a longer stroke without adding much length to the shock, you might retain the angles.

IIRC (and would apply to any frame as long as stuff doesn't clash)

Running a longer stroke is possible in the FR postion is possible, gives a few mm extra travel and geometry somewhere between FR and XC. Bear in mind that a longer stroke shock has a longer body too, so is still slightly longer even at bottom out.

Running a shorter shock in the XC position gives less trvel and slightly slacker angles. Best you could manage would be to achieve the FR geometry with less travel, essentialy you'd have a prophet SL.

If the eye to eye is the same the only problem I can see is rear tyre contact with seat tube and possibly being a bit wallowy?

Thanks this is not a spoon, so a pretty self defeating exersise then?

OK, If you run a 200mm i2i shock with a 57mm stroke (the stock is 50mm) you'll only be able to run it in the XC position as in FR you'll get tyre buzz off the seat tube.

Don't know how much extra travel you'll get, maybe up to an inch extra?

still have fairly steep angles for a 160mm bike, even if you stick 160mm up front, or of course, run it soft and floaty

I know of a few people who ran a 200×57 on some of the old Giant Reigns, instead of the original 200×50, giving approx 6.5" of travel instead of 6, not a massive difference, but a difference all the same. Didn't hear of it causing any problems wit fauling, but not something I've tried myself.