A few suggestions here that might help.
The IGI entry if it says "about 1779" basically means that whoever submitted that entry probably took a guess from the census cert or monumental inscription, or some other record - so don't count it as fact, just a possibility.
Secondly, the 1841 census often rounded ages down to the nearest 5 for adults over 15, so you could have a five year range to choose from.
I've had a few brick walls that I've gone around by thinking slightly more laterally. Rather than trying to work back through your ancestor, why not take the possible parents and try and trace forward on some possible siblings? You could look for witnesses to their weddings in the OPR entries, informants to deaths who may be listed as a niece, nephew or sibling, or a house or farm name that may connect to one mentioned in your line. Also, have you tried to find a pre-1855 burial register perhaps with your local authority, or mort cloth records (they are at GROS, just not indexed yet)? And it might be worth checking the free online will and testament index at Scotland's People if you haven't done so yet.
Good luck!
Chris