The iperf3 results is suspicious, could you post the exact values from the modem, the whole report OpenWrt gives e.g. in the GUI, and the iperf results? Even with a sync of 31.0 Mbps the theoretical limit is 29 Mbps, so 30 really is beyond possible (unless there is some rounding involved). I would trust the modem's reported sync though, if the modem gets this wrong I would expect the sync not to hold.
Full DSL report. Notice that the actual data rate reported is identical to the numbers of @Mijzelf which is suspicious.
root@OpenWrt:~# /etc/init.d/dsl_control status
ATU-C Vendor ID: Broadcom 177.191
ATU-C System Vendor ID: Broadcom
Chipset: Lantiq-VRX200
Firmware Version: 5.7.8.12.0.7
API Version: 4.17.18.6
XTSE Capabilities: 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x2
Annex: B
Line Mode: G.993.5 (VDSL2 with down- and upstream vectoring)
Profile: 17a
Line State: UP [0x801: showtime_tc_sync]
Forward Error Correction Seconds (FECS): Near: 0 / Far: 8
Errored seconds (ES): Near: 0 / Far: 8
Severely Errored Seconds (SES): Near: 0 / Far: 1
Loss of Signal Seconds (LOSS): Near: 0 / Far: 1
Unavailable Seconds (UAS): Near: 153 / Far: 153
Header Error Code Errors (HEC): Near: 0 / Far: 0
Non Pre-emtive CRC errors (CRC_P): Near: 0 / Far: 0
Pre-emtive CRC errors (CRCP_P): Near: 0 / Far: 0
Power Management Mode: L0 - Synchronized
Latency [Interleave Delay]: 0.13 ms [Fast] 0.0 ms [Fast]
Data Rate: Down: 77.848 Mb/s / Up: 23.120 Mb/s
Line Attenuation (LATN): Down: 9.2 dB / Up: 7.8 dB
Signal Attenuation (SATN): Down: 9.3 dB / Up: 7.8 dB
Noise Margin (SNR): Down: 5.4 dB / Up: 5.0 dB
Aggregate Transmit Power (ACTATP): Down: -6.2 dB / Up: 11.8 dB
Max. Attainable Data Rate (ATTNDR): Down: 100.025 Mb/s / Up: 31.277 Mb/s
Line Uptime Seconds: 97724
Line Uptime: 1d 3h 8m 44s
root@OpenWrt:~#
iperf3 results from Windows laptop, temporarily setup behind double NAT, all ethernet:
iperf3 -c RemoteHostOnTheInternet -i -t30
Connecting to host arch.hazejager.nl, port 5201
[ 4] local 192.168.2.130 port 51615 connected to 195.43.158.202 port 5201
[ ID] Interval Transfer Bandwidth
[ 4] 0.00-10.00 sec 34.1 MBytes 28.6 Mbits/sec
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bandwidth
[ 4] 0.00-10.00 sec 34.1 MBytes 28.6 Mbits/sec sender
[ 4] 0.00-10.00 sec 34.0 MBytes 28.5 Mbits/sec receiver
iperf Done.
These results are fully reproducible and within 1% of the speeds I got with the Telco firmware (in bridge mode, single NAT, see first post). The Telco firmware did report an upstream of 31 Mbps.
Thanks! I agree 28.5 * 65/64 * (1522/(1500-20-20)) = 30.174 Mbps, which is clearly larger than 23.120, but would work for a 31 Mbps sync...
I wonder whether the issue now is interference between Firmware Version: 5.7.5.6.1.7 of the blob and API Version: 4.17.18.6 of the driver and dsl_tool?
Looking at the command line reference maybe iperf3 -c RemoteHostOnTheInternet -i 10 -t 30 would work better ;).
That's interesting. I did an iperf either, and also got 29 Mbits/sec, while my DSL still reports a data rate of 23.120 Mb/s. I also tried the other way, but that capped at 48Mbit. First I thought is was my ISP, as I have a 50Mbit plan, but then I saw I have an 52 Mbit upload at the other side. So that is undecided.
@SvenH
You replied to my old topic. The topic has been automatically closed, so I reply here; sadly I wasn't able to fix it at the time.
OK I promised to get back here. This morning the switch to my new ISP (Budget) was completed with no DSL downtime (both Telfort and Budget use KPN wholesale DSL), only a new IP address.
With the following values reported by dsl_control:
Data Rate: Down: 77.848 Mb/s / Up: 23.120 Mb/s
Line Attenuation (LATN): Down: 9.2 dB / Up: 7.8 dB
Signal Attenuation (SATN): Down: 9.3 dB / Up: 7.8 dB
Noise Margin (SNR): Down: 5.0 dB / Up: 5.0 dB
Aggregate Transmit Power (ACTATP): Down: -6.2 dB / Up: 11.7 dB
Max. Attainable Data Rate (ATTNDR): Down: 97.694 Mb/s / Up: 31.277 Mb/s
I get an iperf3 TCP bandwidth (sustained) of 90.0 Mb/s downstream and 29.4 Mb/s upstream. So, I am happy. I suspect that the stock firmware would have gotten a bit more downstream but I am not going to spend more time on that 
Thank you all for contributing.
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I guess it is official now, the reported "Data Rate" for this firmware/blob/driver combination is rather a approximation 
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