First of all, if your brake pedal has alot of travel before you get any bite you need to look at:
1. Fully bleeding the system with new DOT5.1 rated brake liquid.
2. If the above doesnt improve things much, you can buy an fit Goodrich stainless steel brake hoses which will improve things a tad.
By the way, Im basing my suggestions above on the assumption that your pads have plenty of life left, if they are on metal, then the first point of call is to change them.
I'm not absolutely certain about this and Im sure one of the more expirienced guys will correct me, but I think you should be able to fit the 284mm V6/2.0/2.0Turbo discs on your 1.8 with minimal effort. The thing to work out with those is the depth of your current disc and choose a 284mm disc of the same depth. The other factor is possibly the brake calipers, which you should be able to obtain from various breakers from a 2.0 or a V6 car. I believe the mounting points on the callipers are the same, but dont quote me on that just yet....research for yourself on the difference.
Personally I would reccomend Black Diamond as a disc brand, just fitted a new set front and rear of the combi variety and Im very happy with the performance, Also Im almost shocked at how quiet they are for a drilled and grooved set.
On the pad front, Im running EBC Greenstuff which are good for fast road use, but I hear from the guys on the forums that Ferrodo DP2500 (?) are the pads to go for on the 155.
No comment on power, mine is still standard....Im concentrating on getting the handling back in order on mine first of all