Marciatelli

I agree with Maj_Overdrive, Savox 1258 would be the way to go for throttle!
I could suggest Savox SC-1268MG if you want to have both servos from them same company. 1268 has .13sec/60degree 15kg/cm (~208oz/in) at 6.0V. .13sec is alright for driving, not too fast, not too slow, just fine.

I am a twichy driver myself and for me .10 and .12 sec/60degree is like heaven and... not hell but you get the idea. Going beyond .14 becomes sluggish and kills the fun, and if you go super slow then you have to start turning before the turn to compensate the sluggishness. In my opinion it aint fun.

From my experience the more powerfull the servo (high speed with high torque) the more power hungry it is. 1500 mah NiMH is more than enough for few hours of runnig before charging again. Running 2s lipo for receiver is around 7.4V (3.7V per cell) but fully charged it will be 8.4V (4.2V per cell). If you dont have a high voltage operating rx and servos, the circut is gonna light on fire faster than christmas tree with candles on it.
If your rx can operate with high voltage (2s lipo) but your servos cant, you can play around with BEC's (Battery Ellimination Circuit) that are designed for the job (you have to research servo bec's). BEC is really usefull for using LiPO and just messing around making it compatable.
Yes, LiPO is much more powerful with more power capacity compared to NiMH, and it has much more constant voltage. Other than that LiPO needs a specific charger for it, its easy to deal permanent damage it if the voltage drops too low (lower than 3.7V static per cell) and its more feasible to catch fire when case is damaged/puncture. Is it worth to get a 2s then? Its your call. I like my NiMH to drop voltage, have a normal charger and overall being "I dont give a f*ck" battery.

Werk 5B is a good choice, grab some 25% Byron and it gonna scream! Quite easy to tune too.
Which pipe? I used to run my 5B with 2013 and its good, you dont need anything else. I would go with 2013.
I am not a specialist but this is what I know on the topic.
Both pipes are low-end tuned, and are pretty much similar (not the same tho) in nature.
The difference between 2013 and 2058 is that 2058 is a racing replacement for 2013. iFMAR (international Federation of Model Auto Racing) has banned 2013 pipe around 2010/2011 (I dont quite remember when) because its too loud (I approve haha). Compared to 2013, the 2058 is quiter but because of that it is slightly less efficient with the fuel.
So real talk now. If you bash, go with 2013, its loud, has a nice rpm low range, its slightly less restrictive and enjoy the sound.
If you race go with 2058, its quiter and race aproved.

Mark.