The ES330 also has VVTi which will increase engine performance and fuel
economy in response to the higher octane, which in turn also affects
how the transmission performs.

Engines with a higher compression ratio or with lots of advance on ignition
timing need premium fuel.  Higher compression ratios and lots if ignition
timing advance generally produce more power than a similar engine with a
lower compression ratio or with less advanced ignition timing.  Vehicles
with a distributor-less ignition system like your ES and a knock sensor can
electronically reduce the amount of ignition timing advance if it detects
knocking, which is how they are able to burn regular fuel instead of premium
fuel.

The savings from using less expensive regualar fuel are not as great as they
seem.  For example, I usually use premium fuel in my LS 400, and get around
20 MPG around town.  At $2.50 per gallon for premium, that is $2.50 per 20
miles, or 12.5 cents per mile for fuel.  I have experimented with mid-grade
fuel and get around 18.5 MPG, and at $2.40 per gallon, it costs 13.3 cents
per mile for fuel.  It seems counter-intuitive, but at least for my car, it
actually costs less to use premium fuel than mid-grade fuel.

I have not personally experienced the transmission lag that people have
complained about so I can only provide my guess about what is going on.
That guess is that the "learning" part of the transmission control program
places extra computational demands on the computer, and the delay is caused
by the computer trying to do all the calculations and then activate the
shift solenonids.  If my guess is correct, then the reflash would probably
be an attempt to make the code more efficient or program more default modes
for the transmission under a wider number of conditions.

To answer your question as to whether it is possible for premium fuel to
improve (reduce) the transmission lag, I would guess that the answer is yes
for 2 reasons.

First, the transmission electronic control module (ECM) and engine ECM use
some of the same information like throttle position, engine RPM,
transmission output shaft RPM, engaged gear, torque converter status,
individual wheel RPM, and coolant temperature.   If the engine is burning
premium fuel, the ignition timing would be closer to the default ignition
timing and so the engine ECM does not have to make adjustments for retarded
timing and the transmission ECM does not have to "wait" for results from the
engine ECM.

Second, using premium fuel will produce more power for a given throttle
position than with the use of lower octane fuel, and if the engine is
producing more power, the transmission will not have to decide whether to
downshift and take action.

Bottom line, I personally use premium fuel because it costs the same or
slightly less per mile to use, with the added benefit of a more responsive
engine.

I think it would be worth trying 2 or 3 consecutive tanks of premium fuel
and see if the transmission lag is reduced.

The ES trans "problem", that Lexus does not recognize as a problem, has been
addressed by Lexus TSBs, and the latest (At least I believe it's the latest
one) trans TSB has included a note specifying premium fuel should be used.

At my last service visit the Service Advisor told me that, and I doubted it.
He couldn't produce the TSB that day, and so I was virtually certain it was
BS.

I have, however, since seen the TSB, and it DOES state the requirement of
premium fuel.  It's stated in a note on the TSB.  I've yet to try several
successive tanks of premium to test it. I'm just so used to telling the guy
to give me "regular" over the years, It never occurs to me to say "super".