It may be nice in general, but I still can't figure, why do you need it, OP? Want a cab, call a cab and use it. Since there are no taxis in traffic in GTAO it will be hard to merge with traffic going off-radar.
Modifications? Base model of taxi has pretty balanced specs, so adding turbo won't be dramatic in the term of speed, since you can almost drive it without brake, unless want a full stop or smooth 180. Armor is useless, windows are exposed, max speed is a joke. Custom tires won't help traction. Why not use in-game cab, it costs chicken sh*t and no insurance required. Brake it, drop it.
If you want to start cab service for players, buy Exemplar, Super Diamond, Felon or any other 4-door sedan, mod it and be badass transporter.