Sorry but I cant compete with the advice you have already been given, but if you want to read on please do 🙂
In my opinion waterproofness on an MTB and a road bike are 2 different things
On the MTB I wear an event type shell thing
For MTBing its OK
Its a bit sweaty but not too bad
However on my MTB Im rarely flogging myself 'in the zone' like you do on a road bike
My MTBing is more social and less intense
Even riding alone Im usually stopping to take in the views frequently
On the roadie I just get on and hammer it
On the roadie the eVent just isnt breathable enough
My choice for road is a gilet to keep the rain off my trunk and I let my arms and legs get wet
For as long as Im giving it what for I generate enough heat for this to be OK
So there you go
And can I be the first to use the string vest or plastic bag but not both theory 🙂