RAW ≠ Balanced
You appear to be operating under the misconception that 'following the rules' of DnD 3.5e will result in a game where each character is equally powerful. If you have come here with balance concerns about Fireburst, a very inoffensive and thematic blasting spell, your party clearly hasn't discovered the Real Ultimate Power of spells like colour spray, glitterdust, and solid fog, much less the terrifying possibilities of buff-stacking and minionmancy.
So, that is a misconception. Rules-As-Written contains many things more powerful than other things. If you want each character to be able to contribute equally to furthering the plot, you need to do a lot of asymmetric eyeballing - is the Wizard's ability to fireball more powerful than the Fighter's high AC and Charging tactics? Is the rogue's information gathering as useful as the cleric's Divinations? Etc.
Now onto Fireburst. Fireburst is okay for what it does, but not a powerful spell. It is about as powerful as fireball. It does slightly more damage at low levels, but in a smaller area, and requires the Wizard to be in melee range - and if you are designing good encounters, that should be almost suicidal for the Wizard. What is causing you to think it might be overpowered is actually the Sudden Empower and Sudden Maximize feats. In groups with low-powered characters (which yours sounds like) adding metamagic to spells without paying the metamagic cost (as the Sudden line of feats does poorly, and the Divine Metamagic/Incantatrix/Arcane Thesis feats/classes do much more powerfully) will result in a character that seems more powerful than the average run of characters. The reality though, is that the character could do this with a Fireball for similar results. And according to people who know a lot about the mechanics of the game (Optimizers) fireball is the epitomy of the trap spell - a spell that seems great but is actually not very good compared to the alternatives.
If you still think Empowered Maximized Fireburst is too powerful for your game at level 5, and keep in mind the per/day limitations means it definitely might not be, and that is a call the GM has to make, i.e. you. It's not hard once you ignore 'what DnD is supposed to be' memetics and just start comparing numbers and frequency and opportunity, but that can be an initial hurdle, especially if you're running a low-powered group where those memetics seem (note: seem, not are) to be correct.
Although I can't eyeball the balance level of your wizard without more information about both the party and the rest of his build and how hard your encounters are, I can say for a fact that people vastly overrate 'a single large chunk of damage 1/day' compared to 'lots of smaller shots of damage at will'. Sure, a 70-damage fireburst at level 5 may seem OMG POWERFULZ to your group, but as a 1/day 'panic button' it really isn't as overpowered as it might seem, even if your group has fighters with 10 str and rogues who use archery without sneak attack daamge. Hopefully i've talked about the theory enough to give you a bit of an idea about how you go about working out what is overpowered and what is under, and that balance is relative to your party power level and composition, so go forth and go for it.
Also; to see characters at different levels of power, try this character optimization forum, which has a lot of information about power levels in DnD 3.5e.
Best of luck and such.