The Redmine sync is a very cool feature. I would like to suggest an option for the sync; rounded time syncing. Within TMetric there is the option to have hours rounded of, for example 36 minutes would be 0.75 hours because it is rounded of to the upwards nearest quarter.
When syncing to redmine it would then always use 0.25 increments for syncing;
25 minutes = 0.5 in Redmine
36 minutes = 0.75 in Redmine
10 minutes = 0.25 in Redmine
1 hour and 32 minutes = 1.75 in Redmine
Hope you are willing to add this!