I don't own one, but the L67 has LOTS of potential if you're going to do any modding. If you just want it as a DD, then maybe you should just look at a GP with a N/A 3.8 since the L67 requires premium fuel I believe. Plus, guessing by your user ID you have a TA, and if that TA has a M6 I don't know that you will be getting a whole lot better fuel economy, especially on the highway. Plus, not sure how much the GTP costs, but you have to take into consideration how long it will actually take you to actually save enough in gas to cover what the car actually cost to buy.
If you want a second car just to have then that's fine, but if you want it solely to save gas, then perhaps you should rethink buying one, or maybe look at something with a 4 cyl that will save you alot more gas and start to actually save you money much faster.