To improve Rach Success Ratio,please look for Co-BCCH-BSIC cells in its neighbor.
You can also optimize 3 parameters
1.RACH Min.Access Level
2.Random Access Error Threshold
3.Rach busy threshold.
SDCCH Holding time will increase if you increase Max. No. of retransmissions.
Also CCCH Load will increase leading to flow control in BSC