Buddies... There are quite a few factors that would ensure a total Thrawn victory
a) He has a vastly superior ship, the ISD1 Chimaera at his command. This thing has 60 Ion cannons and 60 Turbolasers at its command for a total of 120 weapons. The Iceni has less than 30 weapons on it. Furthermore, the Chimaera has shields capable of withstanding multipe blasts with the yeild of hundreds of gigatons. The Episode II ICS states that the MEDIUM TURBOLASER on the Acclamator TRANSPORT SHIP is 200 Gigatons per shot. The main guns on the ISD, a battlecruiser will obviously be much higher than 200GT per shot and according to official and cannon evidence of the movies, turbolasers can shoot once every second or so. Lets not forget that an ISD can perform a Base Delta Zero strike and reduce the surface of a planet to molten slag. The Sathanas has 1,000,000 hitpoints, or 1600GT, which means an ISD will kill it in 8 hits. The FS1 tech room says Harbinger bombs are 5GT or 5000MT. They do 3200 damage (from ships.tbl), meaning 1 damage point is 1.6MT. A BFRed is rated at 2100 damage points. Beams do this damage around every 1/6 of a second, meaning the BFRed does 12,600 per second or 20.2GT. A Sathanas has 4 BFReds, giving it a 80.8GT per second volley with all four guns (560GT for the 7 second duration). Even if they could penetrate shields, beams would do only superficial damage to the ISD's hull.
Plus, GTVA ships can only target vessels at maximum 10km. SW ships can hit you from over 50,000km away.
b)TIE fighters and bombers are 1000 times faster than FS fighters and they would perform strafing runs so fast that AAA would have trouble targeting them. They can also swarm the Iceni with overwheming numbers. TIE lasers are in the kilotonnage range and are vastly superior to FS fighter lasers.
c)The ISD's hyperspace capabilities allows it to appear at will in any GTVA system regardless of jump nodes and it can use its vastly superior sensors to detect ships preparing for an ambush across a solar system. A mere nebula disrupted the sensors on the GTVA fleet. In the book Star by Star, the Falcon's, a civilian ship, sensors were able to track the happenings of a battle in a nearby system with no difficulty at all.
d)If we bring in the Shivans, they still can't compare to the Empire. A fleet of 80 Sathanases was considered incredulously huge by GTVA tacticians. The pathetic New Republic was easily able to bring 900 ships to attack the YV fleet at Reecee.
c)maneuverability: FS ships are some of the slowest in Si-Fi. They for some odd reason have a finite speed, not acceleration and this speed is quite slow, below escape velocity (around 216 km/h = 60m/s like the HercII). Even if we replace max speed with max acceleration in Gs, its still pales in comparison with SW or even real life (FS ships are slower than the modern space shuttle). SW capships, according to the Episode II Incredible Cross Sections, can reach speeds of about 16,000 km/h in atmospheres and about 3,500G acceleration in space. Freespace capships have shown to be very sluggish in their maneuvers. While the ISD is slightly sluggish by SW standards, Thrawn would literally fly circles around Bosch and I bet while the bridge crew was dancing to "Can't touch this" too.
I conclude that Thrawn would thrououghly whip Bocsh and the entire Shivan race's booty