Have found an abstract of a good article regarding survival of dogs/cats after thymoma surgery:
The conclusion that you will find at the end of the abstract is: Results of this study indicated that most cats and dogs with thymomas did wellafter excision. Even cats and dogs with invasive masses that survivedthe surgery and the few cats and dogs with recurrent thymomas orparaneoplastic syndromes had a good long-term outcome. Excision shouldbe considered an effective treatment option for dogs and cats withthymomas.

Results of excision of thymoma in cats and dogs: 20 cases (1984-2005)
J Am Vet Med Assoc. April 2008;232(8):1186-92.
Julia C Zitz1, Stephen J Birchard, Guillermo C Couto, Valerie F Samii, Steven E Weisbrode, Gregory S Young
1 Department of Veterinary Clinical Sciences, College of Veterinary Medicine, The Ohio State University, Columbus, OH 43210, USA.

Abstract
OBJECTIVE: To provide long-term follow-up information for a series of dogs and cats with invasive and noninvasive thymomas treated by excision alone.

DESIGN: Retrospective case series.

ANIMALS: 9 cats and 11 dogs with thymoma.

PROCEDURES: Medical records were reviewed. The following factors were analyzed for their effect on prognosis: age of dog or cat, invasiveness of the tumor, percentage of lymphocytes in the mass (percentage lymphocyte composition) on histologic evaluation, and mitotic index of the mass.

RESULTS: All patients were treated with excision of the tumor alone. Median overall survival time for the cats was 1,825 days, with a 1-year survival rate of 89% and a 3-year survival rate of 74%. Median overall survival time for the dogs was 790 days, with a 1-year survival rate of 64% and a 3-year survival rate of 42%. Recurrence of thymoma was observed in 2 cats and 1 dog, and a second surgery was performed in each, with subsequent survival times of 5, 3, and 4 years following the first surgery. Percentage lymphocyte composition of the mass was the only factor that was significantly correlated with survival time; animals with a high percentage of lymphocytes lived longer.

CONCLUSIONS AND CLINICAL RELEVANCE: Results of this study indicated that most cats and dogs with thymomas did well after excision. Even cats and dogs with invasive masses that survived the surgery and the few cats and dogs with recurrent thymomas or paraneoplastic syndromes had a good long-term outcome. Excision should be considered an effective treatment option for dogs and cats with thymomas.