Yoshi is a straight thru, and you will notice some smell without a cat. Performance-wise, you definately will feel a bit improvement on top speeds, it will accelerate faster after the 55mph . 0-60 time will be the same. This is because the pcx vario let the engine go 8000-8500 rpm only after the 55mph, and this exhaust is designed to catch some noticable hp only at this peak rpm area.
If you change the vario weights or change the primary vario to the aftermarket one, you will catch around 2 secs in 0-60mph, because the engine will run at peak power rpm (8000-8500) right from the start, plus, with a good exhaust like this yoshi, the engine will be in 8000-8500rpm right from the start, so the acceleration will be even better!
But, imho, pcx comes well balanced right from the factory, people buy it for best mileage per gallon, not for racing. So it is up to you which way to go. As for me this yoshi pipe is a great looking beauty piece first, that greatly changes the look of my bike. And I just falled in love to this bike, when I saw it in 2011 for the first time

That's why I decided to go for all the money, and just installed the 190cc bbk, that gave it 19+ hp at the wheel (OE is 8hp). Still didn't ride it yet, waiting for a warm weather, I am sure my pcx will kick ass!