Yes, you would have the whole 8K of flash for your sketch, and you would no longer have a bootloader so you will not be able to program it any more through the USB port using the documented Trinket upload method, unless you re-install the bootloader using the programmer.
I don't think RSTDISBL is mentioned in their documentation, but it is in the ATtiny85 data sheet. If you program the RSTDISBL fuse, you will get an extra I/O pin, PB5. You would have to program the fuse after you upload the sketch, and then you will not be able to upload anything further until you use HV programming to reset the fuses. There is info in the link I gave.