The Chilterns interchange on the E-W at Calvert is an idea I've considered in the past, mainly because of the potential for passengers from the South Coast and South West to come up via Oxford (and the links from furhter East, which would likely extend all the way to Cambridge in this scenario) and then pick up fast trains to the Midlands and North/Scotland without the current pain in the arse that is going via London, and I think it would be much quicker than via Birmingham with XC as at present. Partly I also think the XC routes will never really be viable to significantly upgrade (on the non existing mainline sections, e.g. Wales/Bristol-Birm-Derby-EC sections and similarly from the South Coast), and so fast/high capacity transport on what is clearly a long distance inter-city route will never match what is on our core axis (e.g. GW/WC/MM/EC/HS2/HS1). Hence trying to intersect that with HS2 via an interchange.
The benefits to the Chilterns would indeed be small (existing lines to London are highly unlikely to be slower once travelling to the interchange is considered bar those that actually are local, which would be a tiny number in the scheme of things), but it would massively "sell the project", not against the determined antis of course, but to the vast majority of people who would feel vindicated and they were getting something. I do fear that such is the (rightful) nature of legal democracy, they will be able to derail (sorry !) or seriously delay/cost inflate the project.
I understand that adding a station impacts on capacity which presumably will be at a premium as with two branches of the Y, the stalk has to take both volumes. However, should a future East Coast branch be built one day to link London direct to the Eastern part of the Y and via some Eastern towns (as HSTed often suggests), then capacity may be released on this core part of HS2.
The one thing I don't quite get about HS2 is the stats the antis have been spreading, that neither Euston or the WCML are actually as busy as other lines, specifically the GWML/Paddington and SWML/Waterloo. Stats of 60% WC loadings and 90+ for the others are often promoted. Are these false/misleading and why ? Granted the GWML is being upgraded to deal with that and Crossrail of course is a huge project, but how does the WC compare to the SW/SC/SE lines for capacity ? - when parked up on a bridge over the WC it was many many minutes between trains - which surprised me as I expected, from the four tracks, a pretty continuous stream. Yet observation on the SWML is much more of a "stream". Very unscientific I know but I'd be interested in the facts.