"By channeling first, spellcasters have a great amount of flexibility - they can draw in as much power as they wish (or are willing to risk) and then choose how to direct that power into a spell." ToM p34

Does this imply that a wizard can cap his power when he channels?

For example: A Willpower 4 Wizard has 4 power points. He is in aggressive stance. He wishes to cast Flamestrike (6 power spell) and so channels. He scores three succeses and gains 6 power - bringing him to 10 power. This is more than x2 his willpower, requiring him to spend a manoeuvre and a stress point to prevent venting.

So two questions I don't believe the text answers:

1) Can the wizard simply choose to stop channeling at 8 power?

2) Is the manoeuver and stress spent in the same turn the power is gained or at the start of the next?

1) No. (But he can choose which success line applies!) Corebook p45

Multiple Success Symbols:

2) At the very end of the current turn (ie after he's cast the spell if he quickcasts) FAQ p7:

Equilibrium
During a wizard character’s End of Turn phase, if he currently
has more power than his equilibrium, he must either perform a
manouevre to maintain this extra power, or loses one power.

Cheers.

I need to remember that 'pick the one you like best' rule.