When are you having the flow issue? Is it directly after you end the boil? Sometimes the wort can still be boiling in the lines causing the pump to have air inside the pump head. This would definitely cause flow rate issues.
Maris is great in in IPA's. Try a 50/50 split with 2 row, no other grain additions. Try a pound or so of corn sugar to dry it out with 5 mins left in boil. Really let's the hops shine.
oxygen caps can have a hop aroma robbing quality when used with IPA's, from my experience. I use oxygen caps on all my beers except the highly hopped ones.