Here is the picture comparing the Geekbench 2 scores between you computer and the current MacBooks. Even the Macbook retina 12 is nearly twice the speed of your system.
By comparing the old CPU on cpuboss
http://cpuboss.com/cpus/Intel-Core2-Duo-T8300-vs-Intel-Core-M-5Y71 I see multiple benchmarks, with the T8300 being much closer on single core in PassMark and 3D Mark 06. (Of course, the T8300 is blown away in AES single core because the newer Intel CPUs have hardware instructions for AES while the older Core 2 Duo has to do it in software).
I am surprised by these Geekbench numbers. No way the passively cooled Core M at 1.2 Ghz is so close to the i5 at 2.7Ghz in the rMBP 13". Maybe for a short spike while Turbo kicks in but not for any kind of load.
Fierarul: What battery do you recommend this older MBP? Lizone is what I see on amazon. I also understand that an SSD could improve battery life. Maybe I should just upgrade this solid laptop, I know for a fact that with 6gb of ram and an SSD I wouldn't have any issues running windows on a virtual machine or even bootcamp.
I did buy new Apple batteries in time for my laptop and upgraded it to 6GB and an SSD. I don't remember it having such an excellent battery life even when it was brand new. But I did stress it a lot with heavy apps. 3-4 hours under light usage I would say.
I doubt the SSD would noticeably help with the battery. It will massively help with the responsiveness of the laptop. Adding an SSD years ago was the reason I've only upgraded the laptop this year (after 7 years of use!). It doesn't even compare with a normal hard drive.
There may be some unofficial franken-batteries to have it last more hours but I never used them. I guess you could also get 2 and swap them but it's a bit inconvenient.
I wonder if you couldn't use a tablet with a bluetooth keyboard at times -- do you really need the laptop for each and every day?