Best bang for your buck is a tuner...I don't know of any mods that will not have potential warranty problems...It all depends if what you installed caused the problem...As far as a total warranty voided, I've not come across that.
There are many good tuners out there, if I had it to do over, I would have gotten the Edge...Lotsa info about that here, do a search and see... Squid has documented the install...Happy to have you here

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7.5 3500QC 6.7CTD 68RFE Build date June 2007
Superchips tuner 3808, Boost/EGT Laser Gauge, Airaid cold intake