That is definately a usable option. While the Focus in Certamen can be mightily powerful, you have to take great care to not have any dump-Arts, where your fabulous virtue lets you add twice the score of 0. A constant bonus is nice, and easier to use. Although this way, the Tremere only have a beginning advantage, compared to his sodales from other houses. As time goes, he will advance in Certamen ability in the same rate as others, assuming a similar rise in Arts. But with the Focus solution, every point in Arts matter more and more as time goes by. And a +3 difference really isn't a lot.
I do see the social implications and problems, in the Order. Because why would everybody else accept a method of solving differences, that heavily favours the Tremere? Nevertheless, this is RAW. Never much of a problem in my Sagas.
Not really used very much, I think I only did it twice. First time in 4th ed a Tremere pissed off my Criamon maga, because he stole some wierd artifact, which could have offered clues to the Enigma. She promtly walked up to him and challenged - this took him off guard. She won desicely in a Perdo mentem duel, and subsequently Black Whispered his arse. Hey, that was legal back then, and it only had Dur:Moon.
Second time was quite recently, in 5th ed. A Mercere was angry that my Tremere blatantly insinuated and genuinely thought the Mercere was lying about some vis use, and that he had stolen it from the covenant's meagre stores, to heal himself of a trivial wound. It later turned out to be true (the consequences of this was recently discussed in thread about punishment etc.). The Mercere demanded an apology and that I stopped accusing him, I in turn wanted a truthful answer if I won. I guess he thought he could win by using his good arts and forcing me to use poor ones. But the duel ended up as Rego Imaginem, and I had all advantages. So I cooked him. Unfortunately I went briefly in Twillight, but with no more concequences than all our species changed person. So to the others, it looked like the Mercere won over me. Had he not been out cold for 2 hours, he would have shamelessly used the fact that he looked (and smelled) like me, and tried to trick the rest by admitting defeat. But I had ample time to redo the Imaginem effects, and make us normal before he woke up, to avoid this.
To nitpick, it can't technically be 'Puissant Certamen' since there is no Certamen Ability or Art. But make it a Virtue granting a flat bonus - like Method Caster - to a certain activity, and this'll make it integrated with the rest of the system. Call it 'Tremere Training' or 'Certamen Mastery', and have it be restricted to House Tremere. Or should it be open to others? This will devaluate the Tremere and remove their 'thing' But is 'Focus in Certamen' restricted to Tremere in RAW?
BTW, back in my 4th ed days, when Certamen was an Ability, Tremere magi often had a Knack with Certamen (info for non-4th ed people: Knacks offered a +1 to +3 flat bonus, for a Virtue costing the same amount). But the main problems IMHO was the use of vis granting +5 per pawn, so even a master of Certamen would be trumped by 3 pawns vis used.