in genarl this is right.
BUT as you for the main factor to allocate the PCH is LA size,(the paging load on BTS is defined by LA size), with calculation of 2th command & IMSI or TMSI plus 20% for peak load plus 100% to retransmit all paging request from BTSs (free resourse on CCCH).
thus the increase in SDCCH by HR implementation hasnt direct effect on PCH. if you use ( MS attach/detach, periodic registeration) with correct PCH dimenssion & robust RF ch.
And furthermore Adapptive confiuration for physical channel to avoid all congestion.