According to MINI2 the 2005 MCS Cabrio will sport not 163hp but a full 170hp to make up for the weight penalty associated with the open top. Here's an excerpt from the MINI2 article:

MINI2 originally reported this rumour in early February, and we're now as certain as we can be without official confirmation the Convertible Cooper S will come with a maximum power output of 170 bhp and a healthy 220 nm (160 lb/ft) of torque. This compares to the standard cars 163 bhp / 210 nm figures. The reason, we understand, is purely to counteract the increased kerb weight of the soft top car, to keep performance figures healthy.

The power increase would be purely a result of an ECU “tweak” at the factory, and one has to wonder if MINI will introduce this small increase across the range (unlikely according to our sources).

You can read the entire article here.

I've talked with a few people at MINI about this and they seemed unaware of any change of hp to the MCS cabrio. However that doesn't mean it won't happen. When Paul from MINI2 says he's almost positive like he has here then I would expect it.