Long term effects will be nil, the extra circumference won't make a difference to the leverage, that changes scratch by scratch anyway, whether you are moving the platter fast, slow, gently or firm... so no changes there in terms of stress on the unit, they are engineered to deal with different DJ styles and techniques, not to be operated with exactly the same force every time from every DJ, so this isn't a measurable factor or an issue.
The weight is almost identical to the 7" perspex platter, the modification 8" or 9" platter weighs less than the 7" record with adapter option you can use on the system as standard, so the engineering tolerances are already able to take this mod.
Both good points to raise, but both answered confidently as not an issue :-)