Someone, apologies for not remembering who, posted his ruleset for "Ars Magica 6" in which all virtues were Major, including House Virtues.
Someone else, again apologies for not remembering who, posted that Certamen would not have gained acceptance as a fair means of settling disputes if House Tremere was obviously disproportionately better at it, since not only would Tremere magi have an unfair advantage, but that Certamen being a fair reflection of magical power that might actually be brought to bear during a real conflict would be undermined.
Someone else, I suspect it was Chris but a third round of apologies if I atrributed this wrong, posted something about the Tremere notion of tines being less than efficient.
So, a new virtue for Tremere magi only
Major Hermetic Virtue: Tines
Choose two Forms and two Techniques. In each of these Arts, you get a 25% bonus to gained experience, rounded up. This acts as an Affinity in these Arts, so you may not also take an Affinity for these Arts. You also get +1 to each of these Arts, which acts similarly to the bonus gained by Puissant Art. If you take Puissant Art for any of these Arts, the bonuses stack. Finally, you roll one less botch die for any roll including one or more of these Arts, and this can reduce the total number of botch dice to 0. You must be a member of House Tremere to take this virtue, which counts as your Major Hermetic Virtue.
Of course, using this virtue means that Tremere now don't get to take an elective Major Hermetic Virtue! Still, this is a rather good virtue. The four lesser Affinities are worth around 2vp. The +1 bonus to 4 Arts is probably worth 1vp. This package would be slightly lackluster, but reducing botch dice for spellcasting (and Certamen!!) down to 0 is an uncommon and valuable benefit.
Unless every House gets a free Major Virtue, adjustments need to be made to fit this in. One way to do this is to change the free House Virtue to Good Parens, and offer this as an optional virtue. Or, have Tremere magi pay 2 extra vp. Or, roll this into a Minor Hermetic Virtue which provides Tines but which also requires Tremere magi to spend one season per year serving the House, usually represented as Exposure xp with nothing else to show for it.