I've got the 8HP45 in my F10 535i. Its doing well at 450rwhp/tq.
I don't think a billet pan would be of any added benefit on this transmission, or a aux cooler kit.
The only faults I've seen on the net is clutch slippage on upshifts with power levels in excess of mine. Pedaling the upshift a bit and getting back into it negates the slippage, but upgraded clutches would fix that problem I'd think. Problem is I can't find any vendors that make upgraded clutch packs.
I also know of an F30 335i with the ZF 8HP45 making almost 600whp/tq with no trans issues that I know of. Nathaniel here in Houston although it doesn't look like hes in here.
8HP45 is ~9 litre capacity, and the filter is integral to the original oil pan. Fabrication of a billet pan would be interesting, likely requiring the sacrifice of an OEM ZF pan as a filter donor to carry over to the billet pan.
Of course if the billet pan is deeper that would cause interface issues with the filter not reaching the mecahatronics, or the bottom of the pan. New filter fabrication entirely? Too much liability for a pan manufacturer to assume IMO as these are spendy transmissions.
The 8hp is a 5 clutch/4 planetary transmission. It nets "8 speeds" by engaging different combinations in parallel. Pretty damn ingenious.
1st gear is clutches ABC, 2nd ABE, 3rd BCE, 4th DBE, 5th BCD, 6th CDE, 7th ACD, 8th ADE.
7th is a 16% overdrive, and 8th is a 33%
It also is able to achieve full torque converter lockup in all 8 gears, unlike its 6hp21 predecessor.
The only faults I have been able to find with it is clutch slippage, and even then it seems to be isolated to AWD models.
Also, I've read the trans oil pan from a 845RE Jeep Grand Cherokee is a direct swap and increases oil capacity.