As a regular traveller between Manchester and Birmingham, both peak and off peak, yes, it needs a hell of a lot more capacity!
The voyagers are packed solid throughout most of the day, with people standing for the entire journey not an uncommon sight. The 30 minute frequency is just right, to be fair, the journey time isn't *that* bad, they just need extra carriages. Then again, so does the rest of the country!
Birmingham - Manchester should have transferred to the West Coast along with the Birmingham - Scotland services, and be run as extensions to the Euston - Brum/Wolves services. Even if it was one an hour, that would still be more capacity than the current 2ph.
Admittedly an 11 car pendilino would be overkill, it'd be a much better situation than 4 car voyagers. Given that the M6 is so busy between the Midlands and North West, there is potential over time to start filling a pendo if the journey was more pleasant.