The plenum spacer (for pre HR cars, if it's got 2 intakes it's a 07 onwards HR) is a non-question really, for the £150 ish to buy and a 30 minute fit for something proven to give gains? Just do it. Do a search for Wasso's plenum spacers on here, probably the sexiest ones around and a good price.
BTW, the K&N typhoon will actually lose you a couple of horses, however you probably won't notice the power loss so if it sounds better a lot of people think it's worth it. The best results for power is the standard air box with a better filter, with a vortex thingy (someone must have the link and proper name for what I mean!) added inside the air box if the car doesn't have that as standard.