A Webra 40 would not / could not get it off the grass!!! The Jett 50 has no problem at all.
The Jett has 25% more power than a .40, based on displacement... The webra .50 would not compare to the jett but would perhaps allow you to get airborne
There are some videos around, showing a t-38 prop pusher with a webra .50. Worth a look.