Will it fit a 2008 328xi coupe?

No. This kit only fits non xDrive E9X models. The xDrive models have a different front axle suspension which makes these control arms incompatible.

I have a 2009 BMW 335i conv. e93 will this fit? Also would I see more of a difference in handing/ stiffness with a m3 control arm upgrade or sport springs and sport shocks?

Yes, this control arm kit will it your E92 335i. The purpose of this kit primarily is to get M3 front alignment specs and to remove the softness of the stock control arm bushing. While the bushings do provide a certain level of stiffness in the front suspension it's the shocks and springs that ultimately provide the firmness of the suspension.