RE: helical or straight offset fletchings
I shoot my goldtips with a 4 degree offset and it puts my Muzzy's right where I aim them.
IMO an offset of 4-5 degrees will give you a good compermise between your bows top speed and the best possible flight. What's you set up shooting for speed now? If you've got good speed loosing a couple of fps isn't a big deal. If you speed is moderate then a 4-5 degree off set may be the fit for you.