if you car is not set up for it the extra octane could hurt you in the end and damage all of you cats. i used 87 everyday but now i am gonna run 103 octane we have here with my combo.
Generally speaking, run the lowest octane you can without preignition/detonation...
With the mods you have, I'd say you could sure use the octane. What's another 25 cents a gallon? Get used to it, it will probably never go down much more.
No, higher octane gas will not help your car perform better!! As a matter of fact, it can make it perform worse. Many folks are surprised how lousey their car runs on higher octane fuel -- low power, crappy gas mileage, etc. Since high-performance engines use high-octane gas, they think using it will yield higher performance out of their car. High-performance engines use high-octane because they have to, or they'd soon be junk.
Octane rating is only a measure of the fuel's ability to resist knocking or pre-ignition. This means that a higher octane fuel does not ignite as readily as a lower octane, given the same cylinder pressure.
Use the lowest octane fuel you can that doesn't knock. A stock 5.0 is designed to run on regular unleaded. As you get miles on the engine, carbon build-up can increase your compression ratio, or create hot spots, that'll cause knocking. You should step up an octane grade to see if that eliminates it, until you can get rid of the carbon build-up.