I thought Arco tests were even tougher than Kaplan. If you really want to give all the Arco/Kaplan tests, do make sure to leave a few PR tests in the end .. The confidence matters a lot in the final test. I had kept 2 PR tests for the last day .. and that helped my confidence a whole lot.

I also work full time .. and I had around 1 1/2 months of prep before my exam. 2-3 hrs a day till April should be sufficient for you. Try to practise with some real tests .. I know evening is a tough time for getting the tests .. but what I generally did was skipped the essays as I figured that a long day at office would be more than sufficient to sap my energy as the essays do! Even then I couldnt do a whole lot of tests on weekdays .. So I scheduled the test just after a long weekend .. (President's day) .. so I got an extra day off from work on monday to prepare .. I solved almost 8 CAT tests in these 3 days as getting these tests in the evening after a full day's work is tough to manage.

As far as improving your test scores goes .. try to find your weak spots .. (mine was verbal .. especially managing time in RC ) and work hard on those aspects .. For RC and CR try to read actively .. try to find the purpose of the passage as most books suggest .. and try to relate the sentences to the main theme always .. that increases your focus .. For SC there are a few good grammar books - Elements of Style and College Handbook .. and websites like this and sentencecorrection.com etc ..

hope this helps.