The F-106 was maxed at M=2.0 in service.
http://www.f-106deltadart.com/manual...06A%201964.pdf
Shaft109 - The Area Rule had not been formulated when the Lightning was designed (and as the above-cited document shows, the Lightning as we know it was basically designed by 1951) but someone involved was clearly aware of the need to avoid a clash between peak body cross-section and the wing, and knew it would be beneficial to have a near-constant-section body and a swept wing.
It might have been Multhopp, who after his RAE days went to the USA and helped design a bomber with two engines in front of the wing and one behind it.
http://www.nationalmuseum.af.mil/fac...et.asp?id=2663
(Seriously, you could paint the XB-51 blue and green with black crosses and fool anyone.)
However, once the Area Rule
was defined, it was recognized that the cross-section could be adjusted and the Lightning grew its belly tank. I have no idea about the T.4/T.5 but it smells a bit of urban legend.