A 200m extension would be pointless, and the challenges of crossing the busy road and disrupting Braehead's car park would make it more costly and difficult. If you're going to do it, just build the station where it is and provide a covered walkway to the shopping centre. (I'm assuming that you aren't looking at extending to Renfrew - as this would require an extension anyway, it makes sense then to have the station closer to the shopping centre).
I still strongly disagree with you that there's potential for this. I will reiterate - aside from a big ski slope (I don't care how busy it is, it's still just one thing) there is nothing in Braehead that you can't get in the city centre. So why would people go into the city, and then get another train out? They won't. They'll just stay in the city. People who want to go to XScape for the skiing will go there anyway, either by car or by bus.
I'm not sure what a "semi-through service" is, but are you seriously suggesting taking Ayrshire and Inverclyde trains down to Braehead, then reversing and heading back onto the mainline?!? (Apologies if I've misunderstood that). Or do you mean running trains Central - Braehead - Renfrew - Inverclyde; much more sensible, but you still have the inevitable problem of capacity in Central HL.