As the subject says.

Is there any performance gain by exchanging my Hitachi SU setup with a single 40 Dellorto carb?

yes i would also like to know this

I am gonna hazzard a guess and say no. If there is any performance gain it will be at the expense of fuel and smoothness of running.

For the untimate performance you would run one carby per cyclinder with vaired length runners to suit each piston.

I would say the the twin hitachi setup would be better all round.

i wouldnt exchenage it, i would keep the su's. In theory the sus would be more economical and midrange would be better, but 40mm delltoro would be better in hgh rpm/ wot situations.

From my experience with the su's, i prefered the 32/36 on petrol but on gas, i love them .

I would just stick to the twin Hitachis as they are a variable venturi type carb, unlike the Dellorto. The twins should provide better economy.

If I was chasing outright performance I would go for the Dellorto, as by design I believe they are more tunable to meet the needs of a performance spec A-series.

Throttle response from the Dellorto would have the edge on the Hitachis also.

If the DelOrto is like the Weber, then you can tune it all you like, but without a 'ported vacuum' feature, you will get no vacuum advance, & that sucks when you're on light throttle cruise.
Fuel is geting expensive enough as it is without removing a feature that improves fuel economy at light throttle.

Those clever Datsun engineers made a great little engine when they came up with the GX. If you have a good one, then don't mess with it would be my advice.

I agree with the rest of the folks who have replied. When I swapped from the single Weber 40 DCOE to dual Hitachi SUs on the Seven, it was like a night and day difference. Lots more torque, better driveablity, and stuff like that. I want to say the "intake honk" sounds better as well, but DCOEs are known for a sweet WOT rort.