Hi.

>The inband IIP3 for WCDMA is specified @ 10MHz offset.

IIP3 is component's specification not WCDMA specification.
Instead of IIP3, IM sensitivity requirement is specified in WCDMA,
Interferences are defined like followings.
 U1 ; Non-Modulated Interference +10MHz apart from Desired Signal
 U2 ; Modulated Interference +20MHz apart from Desired Signal

For component level design, you have to translate IM sensitivity to IIP3.
If you have Filter limiting IF bandwidth, you must compensate this effect
for tranlating IM sensitivity to IIP3.

> So I ran a 2-tone test w/ following parameters,
>RF1=2150MHz
>RF2=2160MHz
>LO=2140MHz
>First of all, should I have used the 2 tones @ 10/20.5MHz
> so that IM3(0.5MHz) falls within the BW(5MHz)?

I can't understand your question.
What does it mean by "2 tones @ 10/20.5MHz" ?
You mean RF1=LO+10MHz, RF2=LO+20.5MHz, IM3=0.5MHz and 31MHz ?

>Secondly, due the LPF @ mixer output w/ cutoff of 8MHz,
> the output @ 10MHz is much bigger than @ 20MHz.

I can't understand your sentence.  ???

With LPF of fc=8MHz, RF1=LO+10MHz, RF2=LO+20.5MHz,
resulting frequencies at IF port are IF1=10MHz, IF2=20.5MHz, IM3=0.5MHz.
You mean IF1 and IF2 are out of pass band ?  ???

> In this case, how do I calculate IIP3?
> Does "Pin+1/2 x (delta between fundamental and IM3)" still apply?
> Which 2 frequencies should I take the delta from?
> Assuming the Pin is in linear region of the mixer.

(1) Determine substitute RF1, RF2 and IIP3 where all IF1, IF2 and IM3 are with in in-band.
(2) Apply loss due to LPF for IF1 and IF2.

There are other methods.

Which methods you use is your own task.  :-/

sniff something fishy here .....why are u applying tones at the LO port ...shouldn't they be at RF port ...???

If you are interested in just the IP3 of mixer itself
You can do it with one tone as well .~(1dB compression+10dB).

And the equation is still valid ...Just consider the filter and mixer as two cascaded blocks... If the filter is attenuating it is just as good ....
    hope it helps.
    shafkat.