You need to go down the list of diagnosis.
Is it full of fluid?
Is the TV cable PROPERLY adjusted?
Does it shift through all the gears, any better than the others or worse?
What are the pressures?
I'm going to assume you have the fluid level correct.
I'm going to caution you that setting the TV cable isn't a guessing game. Set it so that you have a tight cable, full extension at WOT. Don't mess with it from this point unless you have late/hard shifts that are a symptom of a slightly too long TV plunger spring (which is a good thing IMO).
Next,
If it is still acting weak, put a pressure guage on it. It's been covered here many times. Just go buy the gauge, it's worth every penny many times over even if you only use it once.
Report back with the pressures and we'll help ya out.
Did you contact the vendor? I think that should always be the first place you go to.
Next,
and I don't want to sound harsh but...
You bought a trans including converter for $1400, with a "new" input drum (where are they getting those, because GM doesn't sell them anymore), a hardened sun shell (not expensive) an upgraded band, a valve body kit, a servo (Not usually cheap), fully rebuilt, all new sprags, hardened pump rings, bigger boost valve, etc. etc..
In truth most of these upgradesd aren't expensive. A .500 boost valve costs a trans shop about $4, same for the bigger reverse valve. A hardened sun shell is about $15, Alto band is about $25, rebuild kit is about $80, etc.
However it all adds up, plus the converter, plus the core, and labor, overhead.
Do you REALLY think the trans has all it's advertised to have AND top notch assembly?
I'm doubtful.
You can usually get a pretty good gauge of what they SHOULD cost for the unit by looking at various vendors, QUALITY vendors like you have here on TB.com.