"Playing devil's advocate here (not just with you by the way!), surely Rover spent hundreds and thousands of pounds on devlopement and testing, yet their manifolds aren't tapered, and look a little like previous efforts on here."

Hello XRT_si,

Factory (Ford or Rover) produced intake (or exhaust) manifolds are designed with hundreds of thousands of pounds - yes, but NOT for high power output. Most factory inlets are terrible regarding equal flow distribution. The factory spends $$$$$$ or £££££'s to design a cost effective solution (profit) - not performance. Looking at how Ford, GM, or Opel designed a standard inlet manifold is not how a performance inlet would be designed.
There are of course some exceptions to the rule
Some exceptions being factory high performance versions of an engine.
Even these examples are not always the best.

Just my opinion.