I have found best big game performance with the SMK to come from chamberings that produce a muzzle velocity in the 3000 to 3200 fps range for most calibers. Much under this, expansion seems very inconsistant, much over this and on close range impacts you start to see them come apart if heavy bone is impacted.
The 308 Win starts out pretty slow and down range, the impact velocity will be very low for good bullet expansion. THe SMK seems to need a bit more velocity to get it to expand. Generally, the meplat seems to fold in on itself on impact, then depending on retained velocity, they either punch straight through with little expansion or if they have enough retained velocity, they are forced to open up and mushroom as a conventional bullet will.
My theory is that when the meplate folds in on itself, it actually can act as a protective shell over the soft lead core and unless there is enough velocity to force the bullet to expand further, it won't which I beleive is what your seeing with your results.
Because of this, if your velocity is low, you will generally see a decent amount of penetration before you see any significant bullet expansion resulting in enlarging of the wound channel. On a fox, the bullet will likely be well through the animal before it begins to expand. On larger game you see better results generally if this is the problem. If your velocity is so low that there is no expansion, you will not see improved performance on larger game unless you hit heavy bone to initiate expanision.
Something you can do is get a meplate uniformer and this will slightly increase the diameter of the HP and will promote expansion on impact at lower velocities. You lose a very slight bit of BC but the BC will become slightly more consistant as well. In your case, the improved expansion may be worth the trouble.
For the 308 Win velocity class, you may want to switch to the Berger VLD based on the J-4 jacket in the same or similiar bullet weight. These have MUCH thinner jackets then the SMK and will expand much easier, also, they tend to have slightly higher BC values.
Again, from my experience, you are not seeing anything unusual as far as the performance of the 308 Win. Its a great round, very consistant, very accurate but at extended range, rather anemic in performance at least terminally on big game and your bullet selection needs to be much more focused to get proper bullet performance.
Another option would be the Hornady A-max in similiar bullet weights. Again, a thinner jacket then the SMK and a tip to promote expansion even better then the Berger at longer ranges with similar BC. Only problem is you may run into OAL issues with these two bullets if your using your rifle as a repeater.
You did not mention the ranges you were shooting out to, that will tell alot why your bullets are pin holing through targets are well.