You find 5 biomods restricting? How? Considering half the biomods are rather gimicky or useless and the game is already easy, even on the harder difficulty settings, 5 is actually already powergamey. Here's a breakdown the way I see it.

The choice for the eye biomod, is obviously regeneration. With the small cramped maps and the ability to use any weapon without restrictions makes vision enhancement and the spy drone useless biomods (they were great and useful in the original DX though). Don't need to add anymore mods here.

The cranial biomod also only has one viable choice, neural interface. With the only good eye biomod being regeneration, the hazard drone is a rather redundant mod, even on the highest difficulty setting. Cloak isn't a good choice either because the enemy AI is abysmal and sneaking past them is already easy enough. Once again, don't need any more mods here.

With the skeletal biomod, you actually have somewhat of a choice. Are you going to be sneaking past the bots or bashing them? You won't be using the thermal masking mod or electrostatic discharge at the same time because they are both for different styles of play. Which leaves the aggressive defense drone which is useless cos the enemies with rockets or grenades are usually in wide open areas making them easy to dodge anyways. There is also the bad AI issue making them lousy shots with such weapons. No reason to have 2 of these mods at the same time either.

The arm biomod is also a fairly obvious but once again you still have a choice, but like the skeletal biomod above, there is no real trade off. Strength enhancement is obviously the best mod but if you want bot domination then you don't need the inventory slots to carry the heavier weaponry that you'd be using on bots. The biotox attack drone, once again, is another useless mod that pretty much just duplicates the boltcaster. Maybe the only slot where 2 biomods would be good.

The leg mods like the eye and cranial mods, once again, have mostly useless mods, move silent is obviously the best one. Speed enhancement is nigh useless as the playing areas are so small and you already move fast enough (and have a big enough area) to dodge incoming rockets. The only time I found it useful was is made getting the Hellfire boltcaster a lot easier and that's it. Health leech drone does nothing the regeneration mod doesn't do better. Once again, just need one biomod here too.

And there you have it. With so many useless and redundant mods, why would you want to install more then 5? The only thing constricting are the number of good mods available.

Unfortunately, no SDK was released for the game, so mods are virtually non-existent. As far as I know, the only mods for Invisible War are a couple of texture mods (See the Unified Texture Pack.)
I think that's it.

There are a couple of useful utilities. The Deus Ex Widescreen Utility does pretty much what the title suggests. It allows you to play in widescreen and adjust your resolution, FOV, and HUD size easily.
There was another utility that allowed a lot of tweaking (like removing parts of the HUD) called "DXTool" by kefren, but it seems to have vanished from the internet.

Some other unfortunate things is that, that mod as far as I know does not retexture the whole game. (Just the textures used in the first city - Seattle, and several of the main characters.) Also as far as I've heard that mod can only be used on the retail version of the game, it won't work on downloadable versions.

Edit: Ok, I might be a bit wrong about how extensive the texture pack is, I was just going off my memory of it.

Do you mean this?
Does it conflict with widescreen utility?
Is it better than the control panel of the videocard?