No, BS5 wouldn't make him too good. It wouldn't really affect anything (the main reason I don't think we need it).
The real problem with the master engineer now is that he's really good in his primary roll, allowing artillery re-rolls, and completely useless for anything else. 65 points for a master engineer is bargain for a guy who stands next to artillery and gives a re-roll. 65 points for a guy on a mechanical horse is rubbish, let alone 90 points.
This is why they need to re-build the engineer to be a guy who's cheap as chips, maybe just 20 points with the existing stat-line, and then have 'master' as an upgrade that allows the engineer reroll for 45 points. Suddenly, if we have the mechanical horse as a 25 point upgrade, you're only looking at 45 points for a guy who'll charge the enemy on a mechanical horse, or 35 points for an independant character with a hochland long rifle, and so on.
It opens the door to have engineers with wacky guns and gadgets, who have a roll in an army other than servicing artillery.