The spell would confer exactly what it says.
Your base attack bonus becomes equal to your character level
As a 30th level character (in this case Fighter), your BAB would become +30. You would still only have 4 attacks a round however, but considering AC of epic-level monsters, that is a decent buff.
**Point Breakdown**
- You do get a +30 BAB from the spell; +5 EAB doesn't apply. The spell overrides BAB/EAB derived from class levels.
- EAB counts as BAB for the purpose of qualifying for prestige classes and feats and etc. EAB doesn't apply to gaining more iterative attacks. Yes, the spell would change your BAB to +30 for its duration.
- The spell does not specify that it is capped at all. It does specify that your BAB would equal your character level. Dragons are a good example of this. Their BAB equals their Hit Dice. That's why we see 25 Hit Dice Dragons with a BAB of +25. But you are correct that you don't normally gain BAB after character level 20. The spell, however, isn't normal.
**Note:** Any time a feat, prestige class, or other rule refers to your base attack bonus (except for gaining additional attacks), use the sum of your base attack bonus and epic attack bonus. So Divine Power trumps both. And you may substitute both for damage via power attack. Credit: [Annoying Imp][3]