What could possibly go wrong?

Just finished a training run of 150+ km with a ride time of about 6.5 hours so everything is going in the right direction. Last 50km were into a headwind and so I had to up the effort but still kept the pace around 24kph. Two more tests to go and I think I'll be ready. There's a 100 mile event in Bristol in a couple of weeks that will be the last long run and I need to do some hills and that means a trip up the Lickeys (ooh er Matron). I'm now starting to put my kit together and I popped into Evans in Birmingham yesterday. Whilst there I was assured that if I bought a Castelli Prologo short sleeved Lycra jersey with writing on it and a bright fluoro yellow bit then I would definitely go faster. The guy obviously knew what he was talking about as he helped out in the shop on the occasional Saturday and has got a bike. I asked some probing questions just to check I wasn't being ripped off ("How much faster will I go?"; "How does the zip work?") and he was well informed even under this intense pressure. So I coughed up (I cannot reveal the price as he said too many people will then want one and besides Liz might read this) and I am now the proud owner of a new cycling shirt. I didn't use it today as he said I would use up too much of the extra speed and I need to save that for the Scottish Highlands. My plan is gradually taking shape. With forethought like this what could possibly go wrong?