THe GXP version sounds intersting - 270 HP would be sweet. According to at least one article, there were rumors of up to a 300 hp version, but that seems to be getting close to Vette territory, and I am sure GM is concerned about cannabalizing sales. Of course, if the Z4 or S2000 are taking those sales now, it might be a wise move yet.
I do think that having 4 engine options might be a bit much for the Solstice - even 3 seems like a lot. But, perhaps this is the NEW, flexible GM. It is surely something that BMW and M-B have done a good job of - offering 2-4 engine options per vehicle to extend the line across a wide variety of price points.
Doubt that a turbo is in the future. The 2.2 l turbo engine (197 hp) used in the Vauxhall Astre and VX-220 has a cast iron block and is a different engine than the EcoTec.
OTOH the Cobalt has announced a 2.0 supercharged engine so that is available. Also a turbo, unless very small, tends to require more anticipation when driving than a belt driven supercharger which is more "instant response".
Figure on two engines: the 2.4 N/A at 170 hp (maybe 180 hp since Pontiac is an old hand at HP games - in 1970 the ratings had nothing to do with even major cam changes) and 210-220 for the 2.0 s/c. Personally, I would be more interested in the torque curves.
Will make another prediction: the 170 will be available with both automatic and manual trans with bias about 90-10 to the automatic. The s/c version will be available only with manual trans at least initially but will be available NLT 1 January 2006 (and I would expect a very few in 2005 to select purchasers). If production meets expectations then a s/c automatic may become available.
It is not so much what is in the parts bin that matters up front as the cost of certification. Since the Solstice will probably claim the same class as the Cobalt, it may be able to get away with limited/no additional certification so expect the same drivetrain availability for the Solstice as the upper level Cobalts at least until and if sales reach acceptable levels.