mindgam3, trust me when i say that it's a hell of a lot harder for jo to beat your 6P than it is for you to beat his OD. slayer is not the only character that does this and it's a pretty well established tactic against it (may, testament, eddie and a couple others can go through it with their 6P's).
but really, you're slayer and you have ways to bait it, so there's no point in risking getting hit by it if you can avoid it. 6K feint and guard if you're that afraid of it. honestly though, johnny players will usually be intimidated if you're knocking them down a lot, and most smart ones would rather block than to risk 50% meter for something that probably isn't going to work.
as for his combo potential, it basically comes down to four types of combos: low mist finers for damage (midscreen mostly), mid mist finers for knockdown (corner only), ji killer joker for damage (midscreen mostly) and ji killer joker for knockdown (midscreen and corner). if you wanna burst out his jump install combos, try to do it either as he frc's killer joker (that way he spends meter and gets the tension pulse penalty) or right before it (if you fear he's baiting the burst, as the easiest way to do so is to kjfrc and guard). for his mist finer combos, the safest way is to just burst through the mist finer (though of course, as any burst, it can be baited).
when you're knocked down, take comfort in knowing that you can make johnny's life a living hell with, well, any of your reversals really. 2S is a good enough meaty at close range but it's pretty hard to stuff reversals with it. 3HS at a further range is better at it (due to being a level 5 move) but you can still beat it pretty easily. in short, it's a hell of a lot easier for you to get out than it is for him to keep you where he wants you.