You wouldn't see much difference if you were to double the voltage on the SPP or TDI for that matter. The reason is it takes a huge energy increase into the coil for little gain.
BTW, there is no protection or regulation in the SPP or any other White's to protect for too much voltage, so don't even consider it because the results would be disastrous. The TDI might blow a fuse once one of the regulators or other parts blew but the unit wouldn't work after that without repair. The SPP was designed for a maximum of 18V but there is nothing to limit it, so changing the voltage to more would only cause components to fail.
The depth difference between the ML and the TDI series, regardless of which one has more to do with front end sensitivity and design more than anything. Keep in mind that ML has two pulse lengths with one at 250 usec pulse. The TDI and SPP have a 100 usec pulse length. This 100 usec pulse is more than sufficient for most gold found which is a half oz or less. Not much difference would be noticed even if the pulse was extended to 250usec even on bigger gold. In fact, some early GS 5's could select between 100 usec and 200 usec and no one noticed any difference when hunting for gold.
Find and early TDI (through the hole unit) and you can adjust things with a few parts changes or internal pot adjustments. Only a couple of internal pots would make a difference, but most will cause a serious degrading of performance, so even that isn't recommended.
Two big reasons for reduced depth is noise and GB method. The changing of the front end op amp on the TDI helps with noise, but there isn't a satisfactory replacement for the SPP preamp that I have found yet. Keep in mind that the ML costs close to 5 times or maybe more over there than the SPP and 2 to 3 times more than the TDI. At least part of that difference in price is because of engineering to build the best that is practical.
Someone mentioned some time back the difference in depth capability with the GB off between the ML and the TDI was close to less than 10%. With the GB on, the difference can vary with the TDI often beating the most expensive ML under the right conditions. To see what I mean, check out some of the ATX videos where the ML flat misses some gold. These videos were staged to show just what can happen with the wrong setting selection on the ML for the gold involved.
With all that said, the ML is clearly superior but you pay a hefty price difference for that superiority. So, don't expect a Ferrari performance on a Chevy cost.