The need to lower the compression ratio when turboing a N/A car becomes from the fact, that when the engine is given more air (and more fuel), there's a greater amount of exploding mixture in the same given space (cylinder), and when it is compressed the same as the lesser amount before, it has a tendency to self-ignite. This results in "premature burning", which means that the burning mixture hits the pistons when it's still on its way to TDC, which means detonation, which means death to std internals.
One can upgrade to "turbo-grade/forged" internals (or better yet, it's advisable), but there's other things too to do to prevent detonation. One is, that N/A cars have quite an aggressive ignition timing, so that should be retarded. Factory ECU not being tuneable, it should be done with either "piggy-back" ECU, or stand-alone ECU.
Still one way to prevent detonation is to lower the intake air temperature. Relocate the air filter to get colder air, get it heat shielded, install (bigger) intercooler, and/or use waterspray into the intake to further decrease the intake air temperature and to prevent detonation.
So turbo/turbo'd car doesn't actually DEMAND lower compression ratio, it's just one way to prevent the detonation. Audi for example has made turbo engines running ~10:1 compression ratio and still with ~2bar of boost. But as I said, detonation can be mastered by other means, as well. Hope this helps.
#Sami