Assault Gunboats were lame, and besides they never had a very Imperial feel to them.

As for speeds...

X-Wing: the Official Strategy Guide wrote:

Name/Model#  TIE Bomber
Speed Rating:  80 MGLT

Name/Model#  B-wing
Speed Rating:  90 MGLT

The New Essential Guide to Vehicles and Vessels wrote:

B-Wing
MAXIMUM SPEED: 2,390G/950 KPH

TIE Bomber
MAXIMUM SPEED:2,380G/850 KPH

The B-Wing is faster.

Also, let's look at the armament of the B-Wing:

X-Wing: The Official Strategy Guide wrote:

Name/Model#  B-wing
Weapons:  Three Gyrhil R-9X Laser Cannons (single fire or fire-linked)
Three ArMek SW-7a Ion Cannons (singe fire or fire linked)
Three Krupx MG9 Proton Torpedo Launchers (6 torpedoes each)
Twin Auto-Blasters

That's two blasters on the cockpit, one laser cannon and one ion cannon on each of the three wings, two proton torp launchers in the middle, one at the bottom of the main wing. Now, THAT'S overpowered!!

Well, I understand what you're saying about "not matching the sides" which is what happens in so many games where one side's vehicles are just the same as the others, only in different colours, but there is a rough match to Imps vs. Rebs you must admit.

TIE Fighter matches X-Wing - Only the X-Wing had torpedoes and two more laser cannons and is more heavily armoured (and has shields), but the TIE Fighter is faster and more manoeuvrable to make up for it. 

TIE Bomber matches Y-Wing - both are early bombing craft, both are the same speed, both have strong hulls, but the Bomber has no shields, the Y-Wing does, and the Y-Wing has Ion Cannons, whereas the TIE Bomber has extra ordnance and is more manoeuvrable.

TIE Interceptor matches A-Wing - Both are fast, lightly armoured interceptors, (depending on who you talk to - Imp or Reb - one goes faster than the other) but the Interceptor is more manoeuvrable, but the A-Wing has shields but has weaker armour, but also has missiles which the Interceptor lacks

TIE Defender matches B-Wing - Both are second generation bombers designed with more shields, armour, and weapons in mind. The Defender is faster, but both have shields and comparable armour and manoeuvrability. The B-Wing has a slight advantage in weaponry, however, with more missiles and laser/ion cannons.