The HP-1 does about as good a job as it gets with Simulcast systems. I am in Montgomery Co MD and it comes in fine, except when I am directly below one of the towers near my home.
Your issue with Balto City and County may be because of distance from the closest tower, or inter-mod between two towers. P25 systems use much less transmit power than previous ones, and that creates problems from signal loss. If you are between sites, try turning on "ATT"enuation while locked on a talkgroup and see if that makes it better or worse, by selecting the stronger site and eliminating the weaker one(s).
I didn't have any problems on my last trip through Baltimore with the HP-1 running off my GPS, except that I kept getting cab companies because I inadvertantly had Business selected. I was getting Woodlawn clearly while I was over by Towson and Hunt Valley.
One more thing: try refreshing your RR database and re-selecting the city and county if you are using "favorites".
With the switch to re-banding and the new P25 transmitters, there is a possiblility that this might be part of your problem if your favorites groups are more than 3 months old. (also new talkgroups have been added to the database).
Hope this helps.
JK