According to these rules for Revised Axis & Allies I found online, Heavy Bombers do 2 dice of damage, with an expected value of 7, not 10. (If you're getting 10 damage per round per bomber, the dice love you.)
Heavy Bombers are, hands down, the single best technology (with Rockets or Long Range Aircraft being a close second, but only in the right circumstances). Strategic bombing is usually pretty iffy (since each die of damage is fairly pricey): a bomber costs 15 and deals 1d6 IPC damage with strategic bombing, but gets killed by AA 1/6 of the time. The bomber costs 2.5 IPCs per turn (if it lives for 6 turns of bombing), and deals 3.5 IPCs of damage per turn. But with Heavy Bombers, it still costs 2.5 IPCs per turn, but deals 7 IPCs of damage per turn – and now strategic bombing is overwhelmingly valuable: build enough bombers to keep your opponent's income at 0, hold the line with infantry, and win with attrition (assuming that your opponent doesn't have too large of an advantage of units over you).
This other question, based on the non-revised Axis & Allies rules, handles the same issue, asking for house rules to rein in Heavy Bombers – they originally gave three dice while attacking, as opposed to the current two in the revised rules. (The original rules also didn't let you pick which technology you were developing for – you'd roll dice, and for every 6 you rolled, you rolled again to see which tech you developed. So Heavy Bombers required far more luck to develop.)
To answer your question: yes, Heavy Bombers are still a bit overpowered, but buying technology is expensive and variable, so you risk losing battles elsewhere. Most other technologies give you about a 50% boost to something, but Heavy Bombers gives you a 100% boost.