I did a run through of the game with the limitation of only using bound weapons (once I got to the point where I could acquire them) and during that play through, I noticed a few issues that bugged me. Running out of arrows, weapon timer running out mid-battle, some daedra being too high level for banish to effect, low weapon damage compared to normal bows you can strengthen through smithing, etc. So I made this mod which changes some of the attributes & features of the bound bow and bound sword as follows:

BOUND SWORD:
- spell duration increased to 1 hour
- banish daedra magnitude increased from 50 to 100*
- soul trap effect increased from 5 seconds to 10 seconds*
- base damage of sword increased to 2x normal
- base casting cost = 100 magicka

BOUND BOW:
- spell duration increased to 1 hour
- banish daedra magnitude increased from 50 to 100*
- soul trap effect increased from 5 seconds to 10 seconds*
- base damage of bow increased to 2x normal
- base damage of arrow increased to 1.5x normal
- base casting cost = 100 magicka
- number of bound arrows per cast increased from 100 to 1000
- bound arrow speed increased 2x and gravity effect removed

For best results with the bound arrow changes made with this mod, open your skyrim.ini file (default location: documents > my games > skyrim) and add the following lines:

[Combat]
f1PArrowTiltUpAngle=0.05
f3PArrowTiltUpAngle=0.05

*banish daedra and soul trap effects still require you to have learned the appropriate perks

There are 3 files. The main file has both the bow and the sword, the 2 optional files are each of them individually in case you do not want both.

TO INSTALL:
Add the .esp file(s) to steamapps>common>skyrim>data
Add the .pex file to steamapps>common>skyrim>data>scripts

TO UNINSTALL: delete the following files
Better Bound Sword.esp
Better Bound Bow.esp
Bound bow tweak - readme.txt
BoundBowEffectScript.pex (scripts folder)

Latest version: 1.0 (Sept. 17 2012)
This is the first version of the mod, and bar the unlikely chance that someone finds a bug in it, it will be the only version

If you like the "fast/no gravity" bound arrow in this mod, you can find similarly edited versions of the other vanilla arrows in a mod created by Graxster here:
http://skyrim.nexusmods.com/mods/686

CHANGELOG
Added v2.0 which has around 3x base, which is 50% higher than the damage amount used in v1.0