With regards to THK type rails, yes, most homebuilt machines are way overbuilt. 20mm rails can handle some serious loads. Even the 15mm's can. But I think when people see how small the 15mm rails are, they think that they're just not big enough.
As for the ballscrew, are you sure it was a 5mm pitch screw? That's 3000 rpm at 600ipm. Our machine at work uses screws with about a 25mm lead, so the screws are actually turning quite slowly most of the time.