Overlining a 5 weight to say, 6 weight line would certainly have at least *some effect on your ability to cast heavier/larger flies since it would be more line mass in the air to carry the fly. This is common in saltwater for this exact reason (a lot of SW fishermen use an 8 weight with 9 weight line).
If you'll be casting mostly to shorter distances of 30-40', the bass taper line is good for loading and punching/shooting bigger flies out there. I personally don't find the fish-specific tapers to be necessary though, and any regular old weight forward line is fine with me.
I also don't care for the extreme WF lines like a bass taper because they generally suck for long roll casts and don't pick-up or mend worth a damn past that extreme head.
Actually, when I need long casts I tend to prefer a long belly weight forward, or even a straight up double taper line. I find I can pick up more line and maintain the cast better with these lines.
Rods can cast several lines sizes generally, so try it out and see if it meshes with your casting style and conditions.
BTW - any time I've tried to fix a problem by "buying something" I've found that the stuff I already had was fine, but I just needed to improve my casting (and still do after watching Lefty Kreh yesterday in person). For casting the flies you're talking about, be sure to work you double haul well because this and timing has a big effect on casting those dudes because big flies tend to magnify your casting errors
. And mine too!