Please note that I'm not a fan of the WSSM line, but I think you're ignoring the basic metallurgy involved in this equation. The 25'-06 CASE is meant to work around 55,000psi, while the 25WSSM is meant to work around 65,000psi. The powder burned in the 25/06 (longer duration) means it can generate the same velocity, at 55K, that the 25WSSM generates with a shorter burn, at 65K. Barrel length is usually crucial, in this scenario.
Also, you need to understand that you cannot SAFELY subject the case of the 25/06 to the same pressure as the 25WSSM...the case will blow up, and might take your rifle right along with it! By the same token, the very sturdy brass of the 25WSSM almost mandates that you keep pressures pretty high, or else the case won't seal the firing chamber properly, resulting in inconsistent velocity and accuracy.
So, don't try to make one rifle cartridge do what another cartridge is designed for...just use what you have for what it is and don't waste time/energy bashing something else. The inherent problems with the WSSM line will ensure they don't endure, which is evident by the fact the neither Winchester, nor Browning, is offering them in this year's lineup. Also, the enthusiasm shown by the AR crowd will be enough to generate some business for custom builders, but nowhere near enough to save these cartridges.
In order to make a splash in the firearms world, a cartridge has to do something other cartridges don't do quite as well. That can be achieving higher speeds or shooting a larger bullet or generating less recoil. In the case of the WSSM line, they haven't met any of these criterion and what's worse, the problems with how they feed and the reputation they got, early on, for burning up barrels, has pretty much doomed them to failure.
Now, the 25-08 is a cartridge that makes a great deal of sense, to me. The 243Winchester and 260Remington are both successful, (to say nothing of the excellent 7/08 round!) so why wouldn't the caliber in-between work well? My guess is that the 25-08 would offer virtually no improvement over the 257Roberts, and it hasn't sold well in years, so gun companies are reluctant to beat that horse.
Personally, I think the smaller offerings from the 308 family are better than the original because a 30 caliber ought to have a little more powder than the .308Winchester brings to the table...I own a .243 and would buy any of the necked down options before the parent cartridge, including a 25-08!