As I understand it, that preferred network type setting would lock out LTE. So you're not ever using LTE with that setting.
I am still on the original build 2.1.2, and the LTE connection is appallingly bad, but it seems fine on GSM. The big problem is that in the USA, GSM is almost entirely deactivated and LTE is all there is, now.
IIRC there are some mixed reviews here about whether the beta 2.1.5 build improves anything for LTE on the Pro 1x, so I haven't bothered to update yet. Hopefully Fx Tec is still working on an OTA update that fixes the LTE problem for everyone.