Create a new version of Sql.withBatch with support to prepared statements (PreparedStatement.addBatch()). For the vast majority of cases this should perform better than Sql.withBatch using statements (Statement.addBatch(String)).
I've added some patches to perform the suggested change.
The patch files were generated using "diff -u base_file modified_file". Groovy 1.7.5 was used as base.
Additional information here: http://groovy.329449.n5.nabble.com/Groovy-SQL-and-batch-updates-tp4339851p4339851.html