I forgot to mention I stuck a K&N panel filter in, too, so I did notice a slightly better 'pull' when going over about 3000RPM. Might be the fact the engine can just suck in the air from the pipe leading from the front of the car AND the fender well - the old silencer is a massive U bend with a big box attached. The sound was the main thing. Instead of the slight whistle, you get a proper whine, then the sound of the BOV. It's not load like a dump valve, but you get a distinct hiss.
I haven't attached a cold feed pipe, just left as is at the moment. I'm going to find a pipe and try and feed it straight to a place near the fog lamp.
I will just clean the filter a little more often now.