It doesn't matter if it is a 744 or a Dash-8 doing 180kts. They are both going to make the same turn radius.
The "rule of thumb" is pretty good for approach type speeds and a 90 degree int.
If you haven't heard from the controller by 2.5nm, you will shoot thru the loc. You also have to allow for a bit of time to initiate the turn, so 180kt 2nm from loc works ok.
As an aside, having seen controllers at work in Europe, Australasia, Asia, and Nth America...
Heathrow win hands down.
It is a real pleasure working with you lot.