The Filter 8 is a modulation hog...throwing some more LFOs/envelopes at it would be useful. Also, the Filter 8 is pretty much the same thing as the STG Mankato, so it also outputs signals with different degrees of phasing. This makes it useful as a device for generating cancellation signals as well as a VCF. It's worth having, but my choice for ultra-screwy filtering has to go to the Intellijel Morgasmatron. First up, it's a filter pair, sort of like the Sallen-Keys on the MS-20. Secondly, it has some very interesting patchpoints, notably CV over resonance which I'd like to see more of out there. Another "sleeper" filter is Doepfer's A-106-1...another Sallen-Key pair version, but in THIS case it offers an insert point in the resonance path, allowing you to screw around with processing the filter signal ONLY. These pair wonderfully with delays; you could have the 106-1 causing the delay to degrade in quality with every single repeat, for example.
Honestly, you might find the Filter 8 and 106-1 to be a pretty excellent combo, given what each has as their "special trick".