The intermediate servo does play a role in the 2 - 3 shift engine flare. The servo is releasing the band before the Rev/High clutch is fully applied which technically puts the transmission in neutral for a split second. I've found the main cause of 2 - 3 flare is excessive Rev/High clutch clearance. The FSM calls for 0.050 - 0.066 clearance for all applications. I like to have 0.012 - 0.015 maximum per friction disc. Which with 4 frictions will put you at 0.048 - 0.060. The event timing of the intermediate band releasing & the Rev/High clutch fully applying is critical for long transmission life.
In my '68 C4 I have 5 Rev/High frictions clearance is 0.060 a TransGo 47-2 reprogramming kit & a Sonnax C intermediate servo. No engine flare ever on the 2 - 3 shift.