Understanding ADSL Checker Output

I'm trying to understand the output from the BT Wholesale ADSL Checker as it relates to my own situation.

I'm on a relatively long line (about 3km) and achieve 6-7.5Mbps (ADSL throughput) when there isn't FTTC-induced upgrade work on the local exchange/cabinets causing noise on the line...

The BT Wholesale ADSL Checker report for my line states as follows:

WBC ADSL 2+
Downstream Line Rate(Mbps) - Up to 4
Downstream Range(Mbps) - 3 to 7

So I seem to be in the upper part of the Dowstream Range (assuming that means the actual download throughout I should achieve).

For FTTC the report states:

FTTC Range A (Clean)
Downstream Line Rate(Mbps) - 8 (High) 4.2 (Low)
Downstream Range(Mbps) - <blank>

FTTC Range B (Impacted)
Downstream Line Rate(Mbps) - 5.8 (High) 2.7 (Low)
Downstream Range(Mbps) - <blank>

Does this imply that the best I could achieve with FTTC is only slightly higher (8Mbps) than I currently get with ADSL and that there is a risk it could be slower?

Appreciate everyone's help with understanding whether FTTC is an investment worth making in my own situation.

Your interpretation is broadly correct.

One question though: How far are you from your cabinet? That will act as a rough check that the figures are in the right range.

It's difficult to be precise because the cabinet I thought I was on has not been upgraded and the new shiny FTTC Cabinet has popped up in the next village; the same time as my line went "live" with FTTC. However, it's on the right "side" of the Exchange i.e. the new FTTC cabinet is between the Exchange and my House.

I'd say the new FTTC cabinet is about 0.5Km from the Exchange, so assume the line is in the order of 2.5Km from the Cabinet to the House.