rolling p51,
It depends on the type flying you intend to do ..... and the lipos you use.
I have an E-Flite 110 in my Venus II, which will be only slightly lighter than your Showtime 90.
I have also converted a Showtime 90 about 3 years ago.
If you use Lipos that will hold 3.7V under a 65 amp load (like TP Extreme V2 3850's) then you can run 8S and something like a 17x10 prop.
For 15-20C lipo packs, you will need 9S.
Your ST 90 will weigh about 9 lbs with 8S and 10 lbs with 10S setups.
I'm currently running the 110 in my Venus II on 8S TP Extreme V2 3850's with a 3 blade APC pattern prop (15x 13.5) . It pulls 63 amps and puts out 1850 watts. This will give the ST90 "sparkling" performance.
Here's a link to my V2 setup:
http://www.rcgroups.com/forums/showt...t=Venus+II+110

Also, you might have a look at post #129 in this thread:
http://www.rcgroups.com/forums/showt...ht=Showtime+90

Ed