If they could describe what this timetable would look like, how it fits within the available infrastructure capacity and what frequencies and journey times it acheives across *all* WCML journey flows, I would personally be most interested too.
The problem is, even if they manage to find extra paths, it is likely that there are unlikely to be many more.
Based on what has been said the are already 12tph (out of a theoretical maximum of 20), let's say that to allow the railway some capacity for trouble that there needs to be 4tph of the maximum (20%) so that the timetable can recover. That leaves a maximum of 16 paths, 12 of those are already being used, leaving 4.
Of those 4 a couple are (IIRC) in discussion about being used in the next few years. Leaving at most a couple of extra paths to allow for all future growth between now and 2033.
Even based on 2.5% growth per year for 15 years that's 45% more passengers than now when 4 extra paths over the existing 12 paths only gains is 33% extra capacity in path numbers. There could be some additional capacity by running longer (11 vs 9 coach trains on all services) trains.
The problem is for the last 7 years the average (incurring one year where the was a small drop) has been 4.5% per year. If that continue for another 7 years you hit 35% and your extra paths are all used up.
As such it could be that we need to use such paths AND build HS2 to core with passenger growth that could happen between now and the mid 2020's.
One other thing to bear in mind is that the has been limited investment in terms of improved capacity over most of the national network in recent years (I.e. since the ICWC franchise problems), however between now and 2020 there is known to be significant numbers of extra rolling stock being delivered. As such there is likely to be more space for more people to travel potentially increasing growth beyond the current levels. If that does happen then rail growth could be higher than at present and therefore the car for HS2 guess up.
Yes there is scope to increase some train lengths, such as running some 12*23m trains (even if some services have to run as 2*6*23m trains so that they can spilt on route so as to not cause problems at certain stations), but even that is only at most going to gain you 35% more capacity over trains which are 9 coaches. On 11 coach trains it'll be closer to 10%. As such the overall increase is likely to be limited, but again it could put off HS2 due a few years but it would still be needed and now when you try to build it the network is much busier and so it will be more disruptive and therefore cost more to do.