How tall wil this get?

This is my first grow and I have a confirmed female plant that is supposed to be Purple Kush according to my friend I got the seed from. It has been vegging for around 8-9 weeks from seed. I FIM cut it when it was 6-8 inches tall and now it has 4 top braches that are around 2 feet tall. There are also 3 other branches coming up on the outside around 1.5 feet tall. It is in a 3 gallon pot and under a 400 watt MH conversion bulb with a HPS for flowering. I'm am about to start flowing but I am concerned about height. It is alreay huge and it is inches from the light. I made my own cool tube for the light with a glass candleholder, inline fan, and ducting so there is not that much heat coming off of it but it is still a little warm. The tops don't seem to be burning or being stressed. I can not raise my lights more than another foot inside of the wardrobe its in. How much taller will it get during flowering?
I know that they normally can double to tripple in height but I was thinking that since I FIM cut it and it is so bushy and huge, it might not grow as tall. Is this right?

no ..................................................................

okay, from the minute u start ur 12 12 cycle, its gonna do its flower stretch thing for the first couple weeks of bud and slow down after that focusing on filling out with nugz after the prelim stretch. ur gonna c huge gains in hieght. can double. triple. sounds like u dont like that answer...so my recommendation, bein their still in veg...is snap all the bigger branches...ie supercrop. ur already inches from ur light..do it...fold largest growing shoots over, forcefully but delicately. looks like a broken arm would, except itll heal alot faster and stronger than it was be 4...