I haven't heard of too much slope being a problem for the 3 decades I've been around process instruments. For 8 decades, DP transmitters have been piped below a steam line with a vertical impulse tube/pipe. You can't get any greater slope than vertical.

Impulse tube problems occur in gas measurement when solids or gas condensate collect at the lowest elevation. Or in liquid measurement when vapor/air collects at a high elevation point. As long as there's drip/drop leg to collect the liquid/crud in the gas lin and a vent to vent the vapor/air in a liquid line, then you'll be OK.

TuV NEL recommends an 8% slope.