Way back, the DCOE Weber listing for a B18 was a pair of 42's. Very rare and the spec can be covered by 45's. Would mean that 40's would be too small for a hot B20 but OK on a 1.6l engine. The really hot Swede engines tend to use 48s or 50's but they are mega bucks. With the right internals you can get 150-160 with either SU's or Webers. SU's will be right at their limits and need lots of help from the cam and porting. Webers will cope with further tuning. At the 150bhp point you pay for the nice Weber noise with poorer mpg than the SU's assuming that the engines are the same and properly tuned. Engines that Dyno out at 150 can perform very differently on the road or track. The torque usually ends up doing the talking.
You've likely read this
http://www.vclassics.com/mppe1.html
That engine was for sale recently.