As I had the luck to be following the restoration of the 8C 2900B that won Pebble Beach last year, I believe I can share some of Simon Moore's notions regarding Porsche (and add some of my own).
It appears that Alfa realized in late 1934, as production of the 8C 2300 wound down, that a new suspension design was necessary to stay competitive (both in races and in the top-of-the-line sportscar market), and so Jano began a new design for an independent suspension of all four wheels.
There seem to be two Porsche project numbers that pertain to Alfa Romeo (project #63 for the front suspension, and project #69 for the rear suspension), but, while Simon Moore indicated at some point that Porsche sold rights to Alfa, I was told by people doing research in the Porsche archives that there seems to be no documentation about the exact scope of work and/or Porsche's involvement in the suspension design. Then again, Simon mentioned an original drawing dated 01-May-1935 that I have not seen, and to me it's unclear whether that was a Porsche or Alfa Romeo drawing.
The situation gets even murkier when one takes into consideration the "knee action" front suspension design patented by André Dubonnet
and compares it to the 8C 2900 front suspension. Also unclear to me is whether or not, or to what degree, Porsche project #69 related to the gearbox and final drive of the transaxle rather than the quite ingenious 8C 2900 rear axle suspension geometry.
My personal opinion is that the notion of "suspension designed by Porsche" probably stretches too far. I have a hunch that there probably was too much pride in Alfa to completely outsource such an important aspect of car design, and a much more likely or plausible scenario would have been that Jano and Alfa made the initial design, and then approached Porsche to get a second and/or expert opinion about its design -- and for verifying their (Alfa's) calculations.
Let's hope that some day more documentation is found that clarifies what Porsche's exact involvement with the 8C 2900 suspension was.