I have been working with TPIS on throttle bodies. TPIS owner, Myron Cottrell, is a long-time friend; and I disclose this as I am not entirely objective in the following comments.
TPIS modified the LS6 intake to accept an oval 90/74mm throttle body, which produced an additional 18 rwhp on their test CTS-V. I tried it, and it had an anoying intake whistle which was not present on CTIS's CTS-V. There was a feeling of increased power over the entire rpm range.
Then I went to a TPIS ported-stock throttle body. TPIS claims 8-9 hp increase with it, and my seat-of-pants dyno confirms a nice, but lesser, power increase. No driveability problems with over 5K miles.
TPIS has now figured out a way to mount the LS2 90mm intake to the LS6 manifold. I just took delivery of one of these modified intake manifolds and will mount a stock LS2 throttle body on it next week. My guess is that it will increase power in the vicinity of 18-20 rwhp. To mount the LS2 throttle body requires some wiring changes; and TPIS offers a wiring harness for this problem. It is nicely done and looks OEM.
TPIS has a proprietary billet 90mm throttle body which produces about the same peak rwhp as the LS2 90mm throttle body. According to TPIS the billet throttle body has much crisper mid-range.
IMHO, the LS6 manifold matches a FAST intake at most rpm levels on a stock displacement motor. It is also far cheaper. Check out www.tpis.com
. They have printed a magazine article where they extensively tested intake combinations.
Again IMHO, many tuners of LS6 motors overlook the benefits of larger throttle bodies. Once you open up the exhaust with headers/cat back, a throttle body change makes a lot of sense.