When it comes to springs and spring strength there are many more factors that come into play when you talk about “strength”. The quality AND type of steel play a huge part on the strength. Diameter of wire and number of coils also plays a role, but also how tightly those coils are formed affects strength. I have 3 different aftermarket springs for the OOS. All three are different, with variations in strength.
The reason I have this info is because I have made a fixture to remove and replace springs on the OOS traps and have talked to a spring expert on the subject. I will have the fixtures for sale shortly. It will allow anyone to replace springs on any OOS traps, both new and old style.
I know this is an old topic, but Tony makes a good point here. Adding another coil to a spring wouldn't necessarily increase the strength of the spring, and in fact it might weaken it. That extra coil would increase the degree of rotation that the spring could be put through without failing. But yes, there are a lot of factors that come into play. Spring designers have complicated formulas to figure this stuff out. And I don't understand those formulas at all, I just use trial and error.