There are two seperate considerations as to what makes a "hot" load. The first is pressure. You will have no problems here with any commercially available ammunition unless it is defective.
The second consideration is impulse and this is where we worry about the recoil spring. Impulse is a measure of the cartridge's ability to push the pistol backwards. It's not to be confused with recoil.
Impulse has two components. The first is easily calculated as the mass of the bullet times the muzzle velocity. (Don't forget to divide by 7000 to convert grains to pounds and divide again by 32.174 to convert pounds to slugs in order to have consistent units.
Next we have to add the impulse provided by expelling the powder gases and air from the barrel. This is trickier since we don't have an easy measurement of the velocity. There are several rules of thumb, one of which is to use 1.5 times the bullet's muzzle velocity.
So we have ((weight of bullet in grains + 1.5 * weight of powder in grains) * muzzle velocity) / ( 7000 * 32.174 )
If you've done the math right you should get an answer in the range of 0.5 to 0.8 lbf*ft/s for the 7.65 or 9 mm cartridge. (lbf means pounds force, the pound unit got changed from force to mass while you weren't looking.) This will allow you to compare one load to another.
As to what spring goes with what load, I'm still working on that problem.
In general the Luger is a strong pistol and not too prone to breaking. The most common problem is dropping the magazine on its head once too often. The Luger will take a lot of pressure and if you gradually increase the load to the point of destruction you are more likely to break it with excess impulse than with excess pressure. Of course a case full of Bullseye will blow just about anything apart without waitng around for the excess impulse to do the job.
(The difference between genius and stupidity is that genius has limits.)
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