Braking performance ultimately comes down to the tires so I'd probably start adjusting the tire pressure to help maximize grip before swapping pads. As always use judgment.
I strongly advise against running a more aggressive pad in the rear relative to the front. BMW designed a specific brake bias in these cars to maintain stable braking conditions. It's slightly front biased like all street cars; safe, predictable and stable in other words because the front will lock up before the rear.
What you'd be doing using a more aggressive pad in the rear is making the car less stable under heavy braking because you're increasing the rear bias. I've been there; a squirrelly rear end isn't a desirable feeling. The way BMW designed these cars, the brake system is already providing the maximum safe
amount of braking force to the rear axle without locking up the tire (say, enough to get within ~10% of the tire's maximum grip level - this is assuming everything in proportion equal to stock - same tires all around, etc). You'd eat into that margin with a more aggressive pad and may exceed it. Even if you got a rear pad that exactly ate up that front bias safety margin, it probably wouldn't be enough to make much of a difference in overall stopping performance; certainly not worth the risk of making the car unstable. The rears contribute not a whole lot to braking performance anyway so the difference is mitigated further.
For the record I run PFC 08 compound all around on stock brakes. I started with just PFC 08 up front and stock in back, which worked fine. I switched to the PFC 08 in the rear because I went to R-compound tires; they shortened my braking distances, generating too much heat for the stock rear pad to handle without noticeable fade over a session (I totally cooked the stock rears - they were cracked and white around the edges). I didn't notice any adverse effects to bringing the rear pads up to the same aggressiveness as the front.
Edit: good article here, not a ton of specifics but the overall ideas are sound:
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