Mighty Bows greater than +4

[Darklone]

If you want to munchkinize, ehm convert this class to 3rd edition, combine Deepwood sniper with a prestige class in the Quintessential fighter as well as a fighting style there...

This archer does not need a magical bow to shoot dragons.

 

[Xarlen]

Peerless Archer is in the Silver Marches book.

 

[Kae'Yoss]

Is there any way to make a Mighty Composite Longbow +5 or higher within the official rules?

Not directly. But there's nothing there to forbid it. Look up the weapons of the guys in Deities and Demigods. Corellon Larethian's bow has +10 Str bonus.

The bows price follows a simple formula: For every +1 Str bonus, add the weapon's base price: +2 CLB costs 300: 100 for the bow, 200 for the +2. You could expand that, (maybe with a multiplyer above the PHB limits. x2 or even x10, as seen in ELH) indefinitely. a +10 Str CLB would be 1100 gm (1700 or even 6500 bm).