I've just replaced the 30cm flat cable with a 30cm twisted-pair.
I checked it with my cable-tester before connecting it and all looked good. Once plugged in however, the link would not establish. The VMG8924 xDSL LED would flash (yellow = VDSL) rapidly then switch off.
After plugging/unplugging several times I checked it on my cable-tester once again and found that the pair on 3 and 4 (blue and blue/white in this case) were shorted together. Some tugging at the cable to pull it tight has temporarily sorted that out but I'll replace the cable later on (shop-bought cable).
The modem is now sync-ing about 100Kbps faster on the incoming (Down) side.
xdslctl info --show
xdslctl: ADSL driver and PHY status
Status: Showtime
Last Retrain Reason: 8000
Last initialization procedure status: 0
Max: Upstream rate = 1153 Kbps, Downstream rate = 11407 Kbps
Bearer: 0, Upstream rate = 1153 Kbps, Downstream rate = 11088 Kbps
Bearer: 1, Upstream rate = 0 Kbps, Downstream rate = 0 Kbps
Link Power State: L0
Mode: VDSL2 Annex B
VDSL2 Profile: Profile 17a
TPS-TC: PTM Mode(0x0)
Trellis: U:ON /D:ON
Line Status: No Defect
Training Status: Showtime
Down Up
SNR (dB): 6.7 6.4
Attn(dB): 34.0 0.0
Pwr(dBm): 8.2 10.4
So it looks like the combination of going from 200cm to 30cm gained 500Kbps and then going to twisted-pair has added another 100Kbps, so about a 7% improvement.