Don't do it!
6 years ago when I got back into mountain biking I bought a Giro Switchblade, which is very similar. At the time I thought it was great, I had this chin protector bit, and felt invincible. Then I started doing more downhilling, and before long I bought a 'proper' downhill helmet.
Having had a few crashes in my DH helmet, with hindsight, I can tell you the difference between the helmets is night and day. I have put the removable chin guard in the back of the garage, AND IT IS NEVER COMING OUT AGAIN, EVER.
If you crash wearing a correctly fitting DH lid, there is a lot of cushioning between your cheeks and jaw stopping the helmet's faceguard from hitting your face (unless you crash extremely hard I guess, but I think even then it wouldn't do too much damage).
If you crashed wearing the swithblade with the chin guard, there is NOTHING stopping the hard plastic from cutting straight into your face except for approx 1mm of polystyrene - it would damage you as bad or worse than whatever you land on, especially if you landed on nice soft mud. Luckily, this never happened to me *touch wood*.
Happy to take some pictures of it if you still think it's a great idea.