the attainable sync is not a cap so no.
Don't understand your answer ?
Can you set --Maxdatarate higher than attainable, for example say my DS attainable is 35000kbps and US attainable is 6000
would --maxdatarate 38000 7000 40000 work ?
The cap is what the banding is set to.
By default on a new line the banding will match the product spec so e.g. 80/20.
The attainable speed reported by the modem has no relation to the banding, it is simply estimating what the line can achieve on current noise margin and target snrm.
e.g. A line can have an attainable of 110mbit even tho its capped to 80mbit by the banding.
My line is capped to 80 yet its attainable is only 72-73.
There is nothing stopping you set a cap of say 90mbit on the modem, but I would think the DSLAM's own banding would come first and as such you wouldnt sync over 80mbit.