FWIW, I have done 110 mph on a track with the roof rear section up with no damage, so at normal road speeds I think you should be ok. Be aware, anything lightweight (scarves, caps, jumpers, papers etc) will come hurtling over your shoulder and into the passenger footwell just as you are overtaking on the motorway, so stow everything light in the boot.
Personally I always leave the rear section up, as I believe that the better rear view is more important.

To put it down safely and avoid cracking the rear screen, roll up a standard bath towel and place it horizontally against the outside of the rear screen as you fold. This will keep the screen uncreased and so avoid cracking. Drive along with the towel in place. Only put the rear section up again when you get to the beach, at which time the towel will of course be useful for keeping any Germans off the sun loungers

The towel trick is a useful one.

Just make sure the plastic screen isn't pinched and you'll be fine.

If the plastic seems stiff and not wanting to move then give it half hour or so in the sun (tomorrow is supposed to be 30deg) and the plastic should become very pliable. Just make sure you don't see any tight creases - chop them back with your hand if you do. I ran my Chim for almost 5 years and had the roof down the majority of the time without never needing to use a towel - no cracks at all.

Can one use it to good effect on the wife too???

Maybe the odd 500 when they've a mind to

They do less MPG with the back up.. so must have some impact on performance..

You miss much of the exhaust note with the back up surely?

Mine has been down more than up from day one. Sure I could use a new rear screen, but it isn't that bad.
Like they say, just take care when folding and be especially patient when it's cold.

Agree with Lester, back stays up if I'm hooning, down if I'm posing driving at a leisurely pace

I find I get less buffeting from the wind with the roof all the way down and both windows down. With the rear of the roof up, the top of my head is right in the turbulent airflow. Makes my hair look interesting after a couple of hours!

If it's cold or wet then I tend to have the rear section up though.

I don't find any problem with rear-ward visibility if the screen is down properly.
This is how I do it:
First unattach the velcro inside and poke the 'tongues' through to the outside to stop the velcro sticking again.
Then lower the hood down being careful not to crease the screen, push the back right down so that the ali stays almost touch the floor of the parcel shelf.

With is down like this I have very nearly all visibility through the mirror.

Hope this helps.