The Silverliners can, and always could operate on 60Hz AC, though it required a refit kit from GE, at least on the older ones (to compensate for stuff in the propulsion logic, and maybe the MA set?). I believe 25kv taps were standard on the SL II / III cars, but not the changeover equipment. They are on the SL IVs, in fact, a number of them have the required vacuum breakers and tap changers for 'on the fly' changeover. The IVs can run on 60hz - it's a listed operation mode in the book.
BTW, the Metroliners could run on 25kv 60hz AC, too.
There's nothing linking 60hz operation to 25kv - Metro North converted to 60hz in the 80's (in segments), yet kept the voltage at 12.5kv. No reinsulation was required, just replacement of the substations and the needed signal system changes. I think GE sugessted downrating the Silverliners on 60hz, but I'm not sure why that would be the case.
As far as keeping everything 'on' all the time, I wonder what, if any, losses would be incurred. Certainly, there's little current draw, but there are slightly losses in transformers, insulators, etc.
Oh yes, as for running a G on the Reading? I think any rivalry and such kept it from happening. They ran (mostly) fine on the new Haven in the PC days.