The +2 bomber bonus isn't that spectacular. It's actually only slightly relevant, and is middle-of-the-road, as bonuses are also hidden in the Special Force and individual bomber pilot tier classings.
IJN gets +2 bomber ability for nation classing, +8 for Special Force, +4 for Dive Bomber, +3 for Ace DB, and +2 DB Squadron Leader, for a total of +19. If you class a base 10 pilot on time to every class (not necessarily the best way to do it), you come out to 2835 base ability at level 120.
KM gets none for nation and the same +8 for special force, but +5 for DB, +4 for Ace DB, and +3 for Squadron Leader. That's a total of +20, higher than IJN. However, with the lower start bonus, it does come up slightly shorter at 2785 (1.76% lower). This is, however, a low enough disparity that just a few extra vets/experts can make things even.
RM doesn't get the nation bonus, and has the same as IJN for SF (8), but jumps classes first to TB (+4), then just for levels later to DB (extra +3), and gets a jump at Ace to +4, ending out the same. While the bonus is back-loaded, RM gets its classing at lower levels, since it goes to TB, then DB 4 levels later, and only another 14 for Ace DB, capping ability a full 13 levels earlier. It comes just barely short of IJN at 2816 with a +10 pilot at 120 (0.6% lower, a statistical hiccup). Furthermore, IIRC, it has higher crew growth when classed all the way, allowing more vets and experts at its highest class.
SN just blows IJN away. In addition to the +2 nation bonus, it gets +9 at Special Force instead of 8, +6 for DB instead of 4, +4 for Ace instead of 3, and +3 for Squadron Leader instead of 2. That's +24. With that +10 pilot, it comes out at 3187, 12.4% higher than IJN. Note that the SN DB plane is only inferior to its IJN counterpart in fuel and spotting range. It is equal or superior in every other stat.
It does drop from there. MN has a minor bonus, but it's at the end, ending up 2621. The US, having a tree more like the RM one, ends up 2669 with no bonuses. The UK has that hidden +9 at Special Force, but gets nothing after another +5 at TB (again, it goes from TB to DB), ending at a lowly 2562. Of course, UK is the best TB nation by a fair margin, so having the worst DBs is accepted.
However, some players do not class to squadron or even ace as the higher numbers of recruits can be more valuable than the ability bonus depending on several factors. In this case it comes out (stopped at ace / stopped at regular DB):
SN: 3032 / 2820
RM: 2816 / 2584
IJN: 2745 / 2580
KM: 2650 / 2434
US: 2669 / 2565
MN: 2486 / 2274
RN: 2562 / 2562
All told, even here, IJN falls far short of the bonus you'd expect to make up for poor plane durability, while the SN pulls even further ahead.