98 c2500 big block, has slow start, engine cranks really slow sometimes and not others, have 650cca battery (new). Still having issues???? Any suggestions.

P2100 and P2102 refer to the Throttle Actuator not the Throttle body. This is basically the switch system at the actual accelerator pedal. The slow crank and exhaust could be separate issues. Get the alternator tested it might not be supplying enough charge to the battery. Some vehicle OBD systems need to be checked when you fit a new battery. As a battery ages some electronic systems can adjust and compensate for that over time.
If the CAT had gone it would often flag up a O2 sensor fault as the CAT wouldn't be working correctly.
What do you mean by "exhaust noise have increased"? Does it sound louder, Engine's running faster, Sounds rough and uneven? Comment below and I'll try to help you more.

Either 575cca or 650cca, if you are in a colder climate go with the the 650cca.

Sometimes battery cables can short the circuit; try getting them away from any metal or each other.

Aside from building a garage or installing an engine block heater I have used this trick in the past on my 98 RR HSE. It may work for you if the starting issue is simply temperature related. Placing a shop-light or shielded work light underneath your block at night will produce just enough heat in many cases to overcome the slow starting problem.
Other possibilities may be under powered/weak battery. The rovers are known for high cranking amp demand and a weak or mismatched battery could certainly cause this problem. On the outside chance that it may be fuel delivery related you might try priming your fuel injection prior to cranking. Do this by turning key to position 2 (or last position before crank) wait til all the bells and whistles stop usually about 3-5 seconds, and repeat a couple times. If this is helpful, the bad news is it may be indicative of mechanical issues on the horizon.
****SAFETY NOTE***** PAY ATTENTION TO LIGHT PLACEMENT MAKING SURE NOT TO PUT IT IN PATH OF OIL LEAKS.
Hope this helps.

Check your ground from battery cable to ground spot,check fusible links from positive battery cable.

Check the grounds at battery to engine and battery to car body and engine to car body, and check all fuses-fusible links.

Probably time for a new battery. If it is 3-4 years old or older I would lean toward that. Take the truck to your local parts store and have them test the battery out.

if your car have a/c and others i think you need 650cca

TRY THIS BEFORE U SPEND ANYMORE MONEY N PARTS.
Here is the common cause of surges, stalls at stops, slow idle speed, erratic idle speed, rough idle and engine hesitation (and other problems), it is in most cases the idle speed control air-bypass valve and or throttle valve and upper intake, these area's get full of gunk and combustion residue over the miles and cause idle issues (stalls, low idle) like yours, Get a can of intake cleaner from any local parts store, not carb spray, intake cleaner, it is made by a company called CRC, remove the air intake hose to the engine, hold the idle high so the engine won't stall, then spray the can of cleaner into the intake while keeping the engine running, use at least 1/2 the can, shut down the engine and disconnect the battery for 5 minutes, then restart and complete a number of mixed driving cycles, town, freeway, stop and go etc., after a few days the problem will go away as the system will relearn to the clean intake

Make sure you have good clean connections from battery to starter and ground. If all okay, you may want to have a starter draw test performed.