I have both 887VA and Draytek 2850/2850n/2860n+ routers on FTTC without an Openreach modem. I haven't had any reliability problems anywhere. All are on Zen connections.
Give me about a week and I'll test which works at what speed for you. The site with a 2860n+ syncs at 80/20 with maximum attainable shown as over 100/30 (although the 2860n+ doesn't actually show maximum attainable rate, I got the figures from the 2850n previously installed).
I've just got back from there having spent many hours trying to deal with a truly bizarre connectivity problem but the setup is so far removed from a typical domestic one (multiple VLANs, multiple VPNs and multiple firewall rules) that I don't think you should worry about it. I've never come across anything like it before. I've just fired off an email to Draytek to see what they say. Draytek support generally seems to be very good. I haven't needed Cisco support.
The 2860n+ on this connection shows slightly better latency results than the 2850.
It currently looks like this: My 2860n+ Ping
The 2850n on the same connection looked like this; My 2850n Ping
The following quote refers to ADSL compatibility but I have no doubt the VDSL2 compatibility is good too;
The 887VA and E-HWIC DSL interface card were tested extensively with Broadcom, Cisco and BT labs during pre-production development. The firmware is therefore known to offer excellent compatibility, performance and stability across all of BTís ADSL services.
One annoying problem with the Cisco is that the current IOS has an NTP implementation vulnerable to exploitation so it's not possible to use NTP to synchronize the router's clock to an external source. Cisco has no workaround except to disable NTP completely. I'm hoping a patch or IOS upgrade to fix this is available before too long. See http://tools.cisco.com/security/center/content/Cisco...
Other points worth noting; the Cisco has 4 fast ethernet LAN ports, the current Draytek has six gigabit LAN ports. It can also do 3G/4G backup easily (I do that too)
For home use I am currently on a Cisco 857 but will switch to the 877VA (I already have one) when I move to FTTC. I use a separate hardware firewall so won't be using the Cisco for NAT or security.
I'll let you know when I've had time to have a play on the fast connection.