JT, I can't be much help to you because I have no actual experience loading 158 grain bullets in 9x25. I tend to shoot 125 gr. My Lee manual shows a 357sig load for a 147 gr XTP. I would think if you started at the starting loads for that weight bullet (or slightly less) you would be OK. With Longshot, they show a starting load of 6.5 grains.
If you have 9x25 147gr loads worked up these should give you some idea of a starting load. If not you might try some 147gr before playing with the 158 gr.
Do you intend to size the Speer's down to .355 or .356? I shoot .356 in my 9x25 with no problems but have not shot .357. I think it would work but may contribute to higher pressure.
Another thing to consider is that the .357 diameter may cause jams due to the width of the loaded round at the neck. With .356 I have noticed that I can't use too much crimp as this can create a slight bulge which can prevent the round from fully seating in a tight chamber.
If you don't size the bullets down it might also be a good idea to have some freebore off the forcing cone at least until you have worked up loads and OAL. Of course this may already be dictated by the length of your mag, chamber ect.
Let us know what you come up with.
P.S. Nice to know someone is able to get reloading supplies, tough to find around here right now!