I drove my 2.2l Soob in the Speedster about 30,000 miles or so with no rear support. We'll never know what caused it, but last Fall on closer inspection I discovered my left, rear transmission mount was broken completely. The car has 59,778 on it now and I bought it in late 2010 with 8,500 on it. No idea of earlier history.
So, while upgrading to a 2.5l Soob Frankenmotor now, I have decided to upgrade the transmission and add an aux engine mount at the very rear end of the engine compartment. I bought two longitudinal engine mount bars from Small Cars and will build a custom transverse mount from that when I see what's what when the engine is back in. My thoughts are that a rear mount can do no harm and not having one may do some harm or wear out other things more quickly.
In a nutshell...to answer your question...I just don't know. I think the Pro's that are installing Soobs now recommend some extra support.
I'm heading out for Elizabeth, Pa early tomorrow to deliver my transmission to Geno. Shytty weather for traveling but I do like a road trip now and then.