I've had the Mountune upgrade on my 57 reg ST2 for over a year and had Eibach lowering springs fitted at the same time on the glowing recommendation of forum members. They weren't expensive and really improved the looks of the car and its composure. The car no longer lurches through corners and the front and rear don't raise or lower so much during hard acceleration or braking so I would consider the Eibachs an essential upgrade. The Eibachs are Ford approved and have been optimised for use on the standard dampers so you shouldn't have any problems with them.
Mountune have only recently started to release extra performance parts for the Focus ST and we've had to wait a long time compared to owners who have gone down routes such as Dreamscience, Code:Red
or Graham Goode Racing but there are now some good mods out. I already have the Mountune cat-back exhaust which doesn't add any extra power but really looks the part and sounds sublime under acceleration without droning during everyday or motorway driving. I recently ordered the Mountune downpipe and 200 cpi sports cat which they reckon will add up to 20 psi more on top of the ST260 upgrade so I'm looking forward to having these fitted at Mountune HQ on Friday. All of the exhaust components are made by Milltek to Mountune's spec so you can be sure that you're getting a top quality product. Mickyboy just had the full exhaust fitted and he said that the car now responds even quicker and sounds more aggressive.
The general consensous on these forums is that the standard brakes are more than adequate even for mild track use. A big brake conversion is likely to cost around £2000 and the fitting of a Quaife LSD could cost around £1000 so these conversions are a bit out of my price range right now.
The only other thing that would be nice is a cold air induction system but other members have suggested that the reason Mountune hasn't already released one is that it would probably require a reprogramming of the existing ST260 ECU settings to accommodate the existence of an induction system. I remember someone on here fit a K&N Gen II system and found that their Mountune car actually performed worse with it on because the original upgrade wasn't designed with this in mind.
I hope this information helps you out in some shape or form.