In my experience:
In 'soft' ATHR mode the IAS can vary +/- 4kts without the ATHR being overly concerned. At Vapp - 4kts it will give you a sudden increase in thrust, but before that it gives you very little and in small increments. (Im assuming this is a software issue?)
Personally, I find 'bugging up' the Vapp by 5kts, tends to result in a long flare (esp at lower weights around 155T).
The A333 definitely prefers to land heavy (> 175T)
The Trent772 is a large engine and even at 'high idle' (used during approach) it does take its time to spool up. I do find that getting the aircraft stabilised at Vapp by 1000ft AAL really helps with the ATHR stability.
Below 100ft RA, if you apply anypower yourself (I assume this is what you refer to as notching?) you will get THR CLB (not TOGA), even if you you replace the levers into the CLB detent. That is because moving the thrust levers below 100ft RA, disconnects the ATHR.
Lastly & I concur with others here, if the wind is greater than 10kts across, or gusting at all, I personally use manual thrust and fly 2-3kts above Vapp (I dont adjust Vapp in the MCDU, the mini-GS takes care of that)
I reduce the IAS to Vapp depending on the conditions (to stop a long flare), most often at about 100ft.
IMHO:
a. ensure youre stabilised at Vapp at 1000ft AAL
b. use manual thrust if the wind is unstable
c. dont bug up the 'VApp' if using manual thrust (its self defeating) just fly a few nts above and let mini-GS do the hard work.
d. enter the flare at Vapp.
All the above only my humble opinion; Ive not flown the GE engines.