Hi Shimple,
68c is the higher end of the mash temps to produce fermentables (tends to produce flavoursome malty deliciousness rather than thinner dextrinous sugars that are easy on the yeast -I don't know anything more technical than that). I did a basic pale ale on the weekend mashed at 64c... this one fermented out from 1.044 - 1.009 using US-05 at 18c in 4 days (I am leaving it to relax before racking). I was blown away as this is the first time I have mashed so low (and am now more familiar with my setup).
The two brews before that were mashed at 68c and 69/70c respectively (while trying to get to know my set up a bit more intimately - I was aiming for 65c and 67c), and they both only dropped about 25 - 30 gravity points each. I tried the "fast ferment" idea by drawing some off into a grolsch stubby etc but there was nothing left to ferment out...I added a teaspoon of malt out of interest and this caused a good solid ferment within 5 hours so my yeast was fine, but I am positive that due to my higher mash temp the yeast had done all it could with the fermentable material that the mash had produced.
Of course I am inexperienced at all this, but this is one possibility to conisder...(you could try drawing off a small amount like I did to do a mini experimental ferment and that will maybe give you your answer).
Regardless, good luck with it all!