Output transformer type AÜ 6 from Bürklin?

Anyone tried to use that OT in SE guitar amp?
OT is rated at 6W and impedance is 3.2k Ohm/4 Ohm.
I would like to order one from Bürklin to use
it in 5w SE amp with EL84 or 6V6.
I have got one Celestion Vintage 10 and with 8 Ohm
speaker, reflected primary impedance would be 6.4 kOhm.
I know that optimal impedance for EL84 is around 5.2 kOhm, but
I guess that max power with 6.4 kOhm would be just a
little bit lower and clean headroom could be a bit higher.
Am i right?