Yeah, both iPhone 4 variants use identical WiFi chips - the workhorse Broadcom BCM4329. Only the Antenna design differs, but in favor of the CDMA variant (adds antenna diversity - though may be heavily marginalized if at the edge of wifi reception to begin with).
Speed test will show I different icon for wifi vs 3G. Might help you verify which is being used.
I have fios and I thought there was an issue too until I used an android phone to verify they were both slow. On Comcast I get around 20mb down 5 up. Best I got on fios was 14 down and surprisingly 18 up but it's usually around 5 down.