Extending Dunblane services to Perth doesn't necessarily make them less reliable. If the turnaround time is longer at Perth, it might actually improve reliability. After Perth is remodelled it should be a good place to turn trains back and, maybe, stable them overnight for improved early and late services.
Something I often forget about Dunblane-Perth is that the number of track kilometres involved is not particularly high. Since it'd be the first time electrification reaches beyond the Central Belt it sounds like a big deal but it's actually a comparable project to the Shotts line, rather than a grand project like Weaver Junction to Glasgow. The line doesn't have a lot of structures or other complicating factors, so most of the difficulty experienced further south won't happen.