If Sandra Khannouchi coached Art Siemers, I believe he could run about 2:12.51. I base this on several factors.
I feel that Sandra's knowledge of certain pharmacological aids, like um...vitamins...and um...supplements...would be worth about 4minutes.
I think if Art is training full time with no job as Khalid does, Art could cut another 1:30.
I think Art could have run 1:30 faster at Cal if he had been more confident, and it not had been his first marathon. After all he did close the last 4 miles in 20:18.
Lastly, I think Art would lose at least 1:00 due to the fact that his current wife and coach is much better looking than Sandra, and working with Sandra might lower his motivation/morale.
Art ran 2:18:51 at Cal.
Cut 7minutes, and add 1 minute and you get 2:12:51.
I do not think Joseba could break 29:00 for 10k. I think with one month of running I could see a 31:59.
Maybe if he ran exclusively for 4 years, and he had the talent for running, then sub 29 is conceivable.
I think it is possible to run under 2:07 without drugs. I think only a couple of men have done it though.
According to Steve Jones 2:07:13 marathoner, "One can break 2:07 without drugs, but not without Guinness."