Again, point by point.
First, You don't need to do 1000 points of damage against a monster. You just need to remove it, which can be done in a lot of ways. Doing 1000 dmg to 300something HP enemies is just overkill. Shapechange, for example, allows you to easily kill anything present AND, on a standard dungeon-based game, kill the following thing you find. And clean the dungeon. While being able to pass through walls thanks to incorporeity, and ignore traps. With a single spell slot. And because Shapechange gives you the supernatural abilities of the creature, you could just just transform yourself in a choker and be able to cast 2 spells every round. It isn't more powerful because it does more damage (which you do, add together all the damage you did just thanks to the form altering ability of the spell and you can go over 1000 fairly quick), but because of the huge range of options it gives, and the ease of use.
In fact, there are creatures with Wish as a supernatural ability, so you could Shapechange into them and start casting free Wishes.
Moreover, damage is easily avoided, nullified, cancelled, or reduced. Elemental damage, even more so High level mosnters and enemies usually have so many ways to avoid being damaged that they can usually laugh at direct damage. There are so many spell that simply destroy the enemy with no save and no RC that don't rely on damage that you don't really need those blasts.
In addition, standard blasting spells can only be used one time, and have no lasting effect. So, if your enemy survived the attack by means, for example, of readying an action to leave the area of effect, you have lost a spell slot.
Another thing. I'd like to see if your DM really is applying metamagic feats correctly. There are ways of doing those amounts of damage, but I'm starting to doubt he can really do it.
Finally, many other classes are just plainly better at doing damage.
BTW, search "Locate City Bomb" or "Locate City Nuke" if you really want a nuke able to destroy continents. Without metamagic reducers.
Well, I've taken a look at Force Missile Mage. I can't see a single reedemable class feature. Anyway, let me try something a little cheesy. Seeing as your DM doesn't think blasters suck as everyone else here, he would allow this:
Fell Drain, from the libris mortis, is a metamagic feat which increases the spell level by two, and makes the target gain a negative level if he suffers any damage. This could be read (not RAW, though) as, when applied to magic missile, for every projectile the enemy suffers 1 negative level, which stacks with himself. With Twined Spell (+4 to the spell level), that would be 14 negative levels with an 7 level spell. With repeated, it would be 14 levels divided in 2 rounds as a single action as a 6th level spell. And so on.
Negative levels, BTW, are awesome. Yes, more than damage. Far more than damage.
So. Classes. One of the most basic wizards is the following one:
Wizard 4 / Beguiler 1 / Ultimate Magus 10 / wizard +1 / Prestige Class 4, with Practiced Spellcaster (Beguiler) as an early feat or Iluminian as a race. It's plenty good for a blaster, because UM increases the caster level, which is good for blasting, gives free feats, and allows you to pay for the metamagic feats by spending spells from the beguiler list. The fifth level of wizard is used usually to get the alternate class feature Spontaneous Divination, which is great. However, if you prefer, you can chose to not take it and instead pick another prestige class level.
So, if you are trully sure you want Force Missile Mage, you could go Wizard 4 / Beguiler 1 / Ultimate Magus 10 / FMM 5
Alternatelly, good prestige classes for those 4-5 levels are: Initiate of the sevenfold veil, Master Specialist, Paragnostic Apostle, Archmage. Argent Savant helps with magic misiles, too.
For yet another option, You could go Wizard 3 / Master Specialist 10 / Initiate of the sevenfold veil 7
If you prefer a sorcerer, most of them multiclass, but there aren't too many good prestige classes keen on blasting.
Another option for a blaster is a Warmage. Check this guide here:
http://brilliantgameologists.com/boards/index.php?topic=2816.0Feats. For metamagic ones, take a look at this:
http://brilliantgameologists.com/boards/index.php?topic=9876.0Other feats depend on you choice of role, so that depends on you.