Actually, you might have some issues on receive. It's not unusual for someone to connect a handheld antenna to a mag-mount antenna on their car and find that they have to deal with all kinds of interference that the radio didn't receive before with just the rubber duck.
OTOH, a j-pole in an attic already has some reception challenges, so my guess is that you won't notice any problems. If you were talking about a 6 dB gain antenna at the top of a 75-foot tower, my answer might be different.