My expectation is that you did not see any new FTTC stuff but just lots of fibre tubing and maybe bits like some of the stuff at http://www.coolwebhome.co.uk/fibre-milton-keynes/def...
Reason being is that I believe the cabinet you are connected to receive a FTTP based service.
The SuperFastCymru people are the ones who should be telling you about this sort of thing, its what they are paid to do.
What has probably happened is the team doing the core network deployment have moved on elsewhere to do lots of FTTC to ensure that milestones in terms of coverage are hit and will return to install the final fibre manifolds and then the order switch can be thrown. Under Review may simply mean that the schedule for the fibre teams is so busy they cannot be sure when you will be done.
In terms of dropped speeds the fibre kit will not add interference, but may have disturbed some wiring, seeing the full ADSL statistics from before and after might help us to decipher that, but beyond that old IPStream Max exchanges with just up to 8 Mbps services tend to suffer more congestion and as people do more online that is what you are seeing. The fibre roll-out will bring a lot more capacity to area as well as upgrading your access method.