We have no Faccia a Faccia and personally I have no opinion on this feature. What is missing is any secondings ... the more there are, the more chance it gets on a "lets contemplate what to put on a development list" and features what features, such as open / closed, science or general, % growth from reference date, in short same or more than under the team challenge facilities?

Now in order to challenge one needs a way to communicate, if it's a challenge 1 on 1 or open. Present position of WCG is... no email visibility, so how to invite? Maybe through the appearance of a box on the My Grid Statistics page, showing who's as inviting and an email from WCG to member(s) to notify that the member(s) has/have an invitation to challenge?

So, you develop the scheme, work out the features and it might get on that planning to develop list.

Until then, use the member comparison page, take a departure date balance and end date balance and then compute the difference... but, would any individual be able to muster extra power for a bout?