Need Help, Please: Rear Derailleur

[Ajax Bay]

as long it is suitable for the max teeth (32) on the cassette

One further thing: bear in mind that the 105 or Ultegra RDs have a spec of maximum sprocket and a maximum capacity (eg 28 and 37). But a 30t will certainly fit and some eg @tincaman has successfully used a cassette with a 32t largest sprocket, so these 'upgrades' might just cope, in practice (B screw right in).

 

[jefmcg]

Please do answer my original query, I would really appreciate it... can a so called 9/10 speed RD by the manufacturer, be used in conjunction with an 8 speed set up?

They haven't actually answered the specific question that you asked, but have seemingly got to your real question : will upgrading my RD make my gear changing smoother?

The answer to this seems to be "no"