Its going to be a hard one mate but when it comes to stamina a Gyr x Saker has plenty, once they get them wings rowing they can soon knock up a good rate of Knots! the best thing mate is to get him fit and suck it and see, go for it and if he fails nothings lost but if he succeeds you have plenty to gain....
I just found some old pic's of a Gyr x Saker I had off Lenny Walters in 95, I'll get them scanned and put on but he wasn't as big as your bird and topped around the 2lb 5 mark, he could also be a straight liner sometimes if he checked on a pigeon....
I meant to post my bit on your